So I've been reading some Bloodymary (and I'm gonna watch PHM soon so I have the necessary info to write Grace well) but as someone who only watched Iron Lung, the thing that sticks out to me is that Simon continuously believes that Grace and Rocky are with the COI/Eden.
The eel, I can see. It'd make more sense for Simon to believe they're some really weird hallucination from the eel. But just from the fact that
Grace seemingly has no clue who he is and none of the appropriate fear or hesitation that having The Butcher of Eden in the same room as you would cause (like, Simon wouldn't think to give a guy like him that many sharp objects (eating utensils) and immediately turn your back to talk to your colleague(?))
They feed Simon probably more than either group did, and the food tastes amazing (most food rations were probably either flavorless or salty as hell, toss some herbs and spices and that man is done for)
They have a way to make food. That alone would convince him. Grace can synthesize plants and make meat (Meburgers or not, meat is meat and he isn't gonna be picky if it's ethically produced). If he was even vaguely known by the COI they'd be hunting his ass for sport, blood moon be damned.
It'd also make even less sense for him to be from Eden, because he'd at least heard of someone named Grace At Some Point since he literally grew up there, and because his behavior is so not-Eden-like that he'd either not be from Eden or hate it more than Simon hates the COI.
All of these point to him either not being COI/Eden specifically or at least not pro-COI/Eden. Whether he thinks they're another group is another thing entirely, but if he's anti Eden/COI then he's at least not getting their brand of experiments and sacrifices.
It would make sense for him to not whole heartedly trust them, since Rocky is overwhelming at the best of times (Simon would not like that Rocky can hear everything he's doing no matter where he is on the ship/in the biodome), but he also is not the type to go into something already aggressive, especially since right now, his current privileges as a patient are better than his privileges were as a brother of Eden or as a convict of the COI. He only got aggressive with Ava when they were actively withholding information and he was getting desperate for answers, and Grace is definitely not written as the type to withhold an answer to a question.
To me, a realistic first meeting (from Simon's side; once again I have not watched the Project Hail Mary movie or read the book it was based on) would be Simon trying to catch Grace in a lie, not believing him at first but not showing aggression since he doesn't want to lose the privileges he has (he doesn't want to be banned from the plant room, even though he just stands in the entryway terrified his presence would kill them like everything else he's gotten too close to), he sees Erid for the first time, and promptly loses his shit because Oh my god they weren't lying there's life here and food and he can just have some if he asks.
Following that, Simon would probably lean hard into fawning and trying to make himself wanted somehow so they don't get sick of him or "realize" he's just consuming their resources and laying about (Grace doesn't care tbh, having another human to talk to is enough repayment) and would try to get Grace to like him by being agreeable and nice and useful whenever possible (please just give him a job he doesn't care what it is and he won't complain just don't get rid of him please-).
Like Simon is fawning by default because everyone around him was scared of him, seeing someone who isn't scared of him and wants him to live a comfortable life would terrify him since it'd take One Wrong Move to ruin that and he's back to being a convict. (Even though Grace would probably forgive a lot, so long as nobody gets hurt too severely, he'd just need time to calm down).
A realistic Simon is a kicked dog, yes, but specifically a kicked dog that thinks if it barks too loud it'll get kicked again, and his ribs only just started healing.
Going back to the eel, I am a derealization Simon truther. He might suffer from psychosis generally (they can be caused by both trauma and concussions, which Simon certainly has enough of both to qualify), but I think the most pressing problem would be disassociation, since him actually seeing Erid would snap him out of most of it (even the eel couldn't make him hallucinate a planet this green and this vibrant with life, even if it wanted to. The human mind's limitations wouldn't allow it).
I also think the derealization wouldn't actually go away, even after years of living on Erid. He'd probably sometimes pinch himself or repeat mantras or look for Grace if he's not teaching to remind him he's on a planet with life and food and the stars are alive and he's alive and he's not in the submarine anymore.
Also also I think he'd one day wake up outside of a derealization episode and think about caring for the plants and cry because he wishes he could tell his mom how pretty all the alien plant life is and all the constellations the Eridians taught him about and how alive everything feels, but even if all of the fleets made it through whatever wormhole or astrophage or whatever took Simon, he'd never have a chance to tell her he made it, he's alive.
I think that'd eat at him sometimes.
