cryptic and generally subpar tactical decisions

Elena (and, through her, Greil) knew 1) that Lillia wanted the medallion to not be in Daein's possession and 2) that the medallion was very dangerous. I don't know if the game ever says that they knew that Yune was sealed in it, but it took around a decade for Greil to accidentally touch it which feels like it's a bit tooooooo long for him to have gone without touching it without it being something he was actively trying to avoid doing, so it feels like they at least knew that letting anyone but a heron or Elena (and, later on, Mist) be in a position to come in contact with it was an accident waiting to happen.

I think their first choice would've been to bring it back to Serenes (Reyson says Lillia wanted it brought there, and, even if that wasn't possible to communicate through the language barrier, Elena and Greil should've been able to figure out something along the lines of "this object is related to the herons and that means they should know how to deal with it"), but Elena and Greil might not've known that any other herons had survived (Ashnard is surprised to hear about Reyson and Lorazieh being alive in chapter 18 and I imagine that he would've been better informed than they had been at the time). Unable to give it to its rightful owners, they could either give the Incredibly Dangerous Object to someone else for safekeeping or hold onto it and keep it safe themselves. Given that they had just uprooted their whole lives and were now fugitives, they had nobody they could trust so they had no choice but to hold onto it.

They also could've. like. buried it in the middle of nowhere or thrown it into the ocean, I guess? But while that would make it difficult (but not impossible!) to find in the short term, there was no guarantee that given enough time it wouldn't wash up on a beach somewhere or get exposed in a landslide or something and be found by someone who Elena and Greil can be certain wouldn't know not to touch it. It presents the same sort of ethical issue we have with things like nuclear waste and mines in real life (these objects are dangerous and remain dangerous for a long time; how do we ensure that people aren't hurt by them in the far future?), so it wasn't really an option either.

So they take the medallion out of Daein themselves. For countries that share a border with Daein, their initial options would be either Begnion or Crimea, but Begnion had just committed the Serenes genocide while Crimea at least officially recognized laguz as people. Staying in Crimea would be risky, though, because it's right next door to Daein and they could attempt an invasion like they do in Path of Radiance, so it'd be safer to keep going at least until they entered Gallia and could use Crimea as a buffer between Daein and the medallion. (not specifically in a "we're using this country as a sacrifice so we have time to flee if Ashnard launches an invasion" way but more in a "we're too far away for an invasion to be realistic, so it limits how much he can do to get it back" way)

I think if they knew Yune was sealed in the medallion they might've considered trying to enter Goldoa to pass it onto Dheginsea (as someone who defeated her), but that'd be complicated by how Goldoans aren't huge fans of beorc and by how Goldoa maintains pretty strict isolation from the outside world. They'd definitely be barred entry and need to explain what they were trying to do if they wanted a chance of getting in, and I imagine that Goldoa would want them to just hand over the medallion at the border and leave which...? I'd personally be iffy on doing because I'd have no way of proving that these guards are legit or and no way of knowing that it'll actually get all the way to Dheginsea, and that's assuming he's even willing to let an item that's practically begging for aggression from another country be brought into Goldoa. The best case scenario is really good, yes, but it's completely possible that it doesn't work and given what's at stake I would greatly prefer only telling people who absolutely need to know (the jerk guarding the Gallia-Goldoa border does not need to know unless he's going to be letting me in), so it's overall safer to not try to push your luck and just be satisfied with hiding in Gallia.

I think it's reasonable for Elena and Greil to not immediately trust Caineghis when they enter Gallia. They're on the run from Daein, and while he's not obviously evil they still don't know if they can trust him with the medallion because they just met him. It's in no way his fault, and I think that the only people in Tellius that they wouldn't have this initial level of caution around would be Reyson and Lorazieh and that's solely because they're herons and the medallion belongs with them.

Once they did trust him, though, they really should've brought it up since Caineghis has access to a lot more options for dealing with the medallion (or at least learning more about it, since he can get Dheginsea to talk to him). We could probably expect it to make its way to Reyson given what we know, but they would at least be able to expect there to be some options for dealing with it that would more reliably keep it away from Daein than "Elena holds on to it 24/7", right?

The reason why they didn't do that was... probably mostly misplaced optimism? I think that if they believed that Daein was capable of finding (or at least of hunting them down so soon) they would be a bit more prepared for that eventuality, and part of that would inevitably involve speaking to the ruler of the country that you're seeking asylum in about the issue of the medallion. There's no reason to doubt that Greil and Caineghis were close so we can be sure he and Elena had reached a point where it was clear that Caineghis was trustworthy, so the only other reason I can imagine why they wouldn't try to address the issue before The Incident happened would be if they thought the issue didn't urgently need to be addressed.

Maybe they thought they were deep enough in Gallia to never be found, or maybe they thought that Ashnard had given up, or maybe they thought that Ashnard would try to launch a full military invasion to get it (and not send a smaller group of people like he ended up doing) which would give them enough warning to flee again (like Greil does in Path of Radiance). It's fairly understandable, I guess? It had been around ten years and nothing had happened so I can't really fault them for possibly starting to think that Ashnard was focusing his efforts elsewhere.

Greil choosing to not to explain all of that and what happened with the Medallion after he touched it is... uh. Like I get it 100% and I'm not saying that I would've done any better but I think it's a lot harder to frame positively.

Greil should realize at this point that he cannot deal with the medallion all by himself without continuing to endanger the people he cares about (both by having the thing that makes you lose your mind nearby and also by having the thing that Daein is hunting down nearby), which is probably why he eventually ends up explaining the situation to Titania and is definitely why he hires Volke. Unlike Volke and Titania, Caineghis is, y'know, a king. He is equipped to handle the matter of the medallion in ways that Greil and Titania and Volke definitely couldn't and Greil is choosing to endanger basically everyone by not talking to him, so he had to have had a compelling reason to continue hiding it.

From Greil's point of view, after everything that just happened, three main things have changed:

Daein is still after the medallion

Greil (accidentally) killed Elena

Greil (accidentally) killed most (if not all) of the people in the town/village he lived in (the 'if not all' is mostly inferred from how Volke, Titania, and Greil were the only people who knew what happened; if there were survivors, I think we can expect them to have come forward and for more people to know about what happened or at the very least know that Greil was the one who was responsible)

Point 1 is not really a change to Greil's situation, but it's forcing him to admit that he needs to change how he's approached the matter of the medallion. He could've talked to Caineghis, who we know he trusted quite a bit, when he approached Greil seeking answers, but he instead chose to relocate to Crimea because...

Killing Elena would have put a lot of strain on him. I think that even if we remove every other factor and the situation was just that he had accidentally killed her, Greil would have incentive to leave Gallia simply because it is the place where he killed her. How could he bear to live in a place haunted by her memory? In a place haunted by what he did? How could he ask his children to live there? How could he ask Ike to live there, in the place where he watched his father kill his mother? But that doesn't explain why he didn't explain anything to Caineghis before he left, which was probably because...

Killing all your neighbors is bad. A real big faux paus. Even if it was an accident, Greil killed an entire town and no matter how close he and Caineghis were, Caineghis is the king of Gallia before he is Greil's friend. From Caineghis's perspective, Greil knowingly brought trouble into Gallia and that trouble eventually got a bunch of unrelated Gallians killed. He could potentially grant some leniency on account of the circumstances (Greil was not in control of himself), but when it comes down to it what Greil did was not the kind of thing that can get swept under the rug and Caineghis is not the kind of ruler who would try to hide it either.

Greil would face consequences. Gallia only somewhat has a government, but the lack of a formal judicial system would not change the fact that Caineghis would be obligated to judge Greil and, if anything, the lack of a formal judicial system would mean that Greil couldn't anticipate what would happen if he came forward and told the truth. To some extent the decision to leave without explaining anything had to stem from a desire to avoid being personally harmed, and to some extent the decision to leave was fueled by shame, but on top of all that there's the question of what would happen to Ike and Mist.

For the sake of argument, let's assume that Greil confesses and is executed (for the purposes of this exercise we're concerned with how this affects Ike and Mist, so the specific means of removing him from their lives is not important). Caineghis obtains the medallion and passes it on to Dheginsea, Dheginsea deals with it, and the world is saved. Then what?

Ike and Mist are now orphans (which is bad enough on its own) and too young to support themselves, so who will take care of them? Caineghis? Even if we assume that he'd still want to, how would it look for him to have Greil killed and then turn around and adopt (or assist) Greil's children? Caineghis already barely has any power, so would he really be willing to undermine what little authority he actually has just to help Ike and Mist? Would Greil feel like he could ask Caineghis to? And what about Titania? She's old enough to support them (with some difficulty) at this point, but they've only known each other for around fiveish years... even if we assume she had left the Crimean Royal Knights by then, would Greil even think of her as a potential foster parent? Would she even want to, now knowing what Greil had done (without having had the chance to grow closer first like in the actual game)?

(And that's not even getting into the ways that losing both parents in quick succession is going to hurt them emotionally!)

Greil has to answer all those questions of "what's going to happen in the future re: my kids and the medallion and Gallia and this and that and everything else" while grapping with having killed Elena, so I think it's pretty reasonable for him to be making a "bad" choice here. He's prioritizing his safety kinda and mostly the (short-term) happiness/wellbeing of his kids, though the contract with Volke and him choosing to eventually tell Titania about what happened and choosing to live in Crimea much closer to Gallia than to Daein is him clearly starting to be more realistic about the responsibilities that come with possessing the medallion so I don't think that it's fair to act like he's only acting in self-interest. He's still trying to deal with it basically on his own (which is bad because there's no world where he was going to be able to solve the problem himself), but he's being more proactive in planning for the future which is... an improvement, at least?

As satisfying as it is when a character you've been aware of for a very long time finally joins your party (some Gotohs, Elincia, etc.), putting the sovereign of a nation on the battlefield is a massive risk because replacing your sovereign tends to be a very difficult, annoying, and time consuming process (assuming they can even be replaced!).

With that in mind, the fact that Skrimir is allowed to be in a position of potential peril as someone fighting on the front lines as general means that to a certain degree his death is regarded as 'acceptable', right? i don't mean that it was something anyone wanted, but the benefits of being general (experience and chances for exposure/networking) outweighed the risk of having to replace him should the worst come to pass. The game even accounts for this possibility by letting Skrimir die without giving you a game over once you're past 3-P and giving Caineghis a different ending where he isn't allowed to retire until a new successor is picked. That second part (the risks of being general being 'acceptable') is mostly what I'm hung up on, though it's consistent with Giffca (and, through him, Caineghis) not really making a big deal of how many Gallians died when he checks in with Skrimir near the end of Part 3*, so i don't really worry about it too much.

If Skrimir does survive, however, he ends up patrolling Gallia's borders as king which...? He's not leaving Gallia, sure, but he's still putting himself into harm's way, so i have to wonder: if Skrimir is allowed to Do Stuff, how hard and fast of a rule is 'Gallia's King Can't Do Stuff' really?

Now, of course, we don't really know that Skrimir is being allowed to do this and it's not him just doing it because nobody is able to make it have consequences, but if Skrimir is able to ignore whoever is telling him he shouldn't be running around, Caineghis would certainly be able to ignore them if he really wanted to as well, right? The fact that he doesn't then must mean that he was convinced that keeping himself safe was what was best, which makes sense for the person he is at this stage in his life, but when he was younger? I'm not convinced he was doing much listening back then.

a sample route i put together: