what do i mean by 'the two guys canonically driven insane by being informed of the future and their choices not mattering go react poorly to being informed of the future and told none of their choices matter'? well,

they react poorly.

Jevil and Spamton both were already sort of under the impression they already defied fate- Jevil by helping Frisk stand up to King, Spamton by helping Frisk stand up to Queen + the NEO fight where Frisk 'cut his strings' and got him moving again despite it. Spamton far more so than Jevil, as Jevil's simply not been thinking about it in depth, while Spamton. is Spamton. and has been thinking about it for years and years and-

So seeing a prophecy that DOES make reference towards them and Frisk makes them freak out a bit! Were any of their choices even theirs to make? Does any of this matter? If none of it does then-

[I can't do anything] I CAN DO ANYTHING.

And in BT's chapter 4, I think that they're.. .seeing bits and pieces throughout but nothing that really Sets Them Off (because Aarch is deliberately breaking pieces of it like Ralsei does) until they're in the room in which the Knight makes the Titan, where they see a piece of the prophecy that sort of- straw that breaks the camels back.

then the Knight shows up, has a laugh at their expense, Frisk tries to confront it, and it promptly creates the fountain (which stabbing their knives down to make it also breaks the prophecy panel). Aarch Did Not Know About This, It Was Not Mentioned, and Spamton and Jevil are still having their break down so they're IN the battle not taking turns or listening to Frisk.

As for how that Titan fight goes and how its cleared up, well... I'd like to try to draw something proper for that ;]

but also, I want to say, the prophecy panels that feature Jevil and Spamton are actually very ambiguous on whether they HAVE defied fate. They basically only acknowledge the parts where its mandatory to encounter them (Jevil attacks them at the start of Chapter 1, their only path forward in Chapter 2 is through an alley Spamton is in), not the fact that Frisk had them in their party, not the fact they're STILL in Frisk's party.

they're pretty much the same! on friendlier terms with jevil since jevil hasn't lost it (... completely.) though I'm debating whether or not they're still the court magician? I kind of feel like they probably retired or dipped out of it once spade King started acting out. alternatively, they still are OFFICIALLY the court magician and were sent to go collect Jevil after Jevil's wondered afield (and also told to kill the lightner if they happen across them) but is . not doing that. they are taking the chance to ditch and are not actually following King's orders. their shop is a traveling magician's cart.

and they DO give Frisk a heads up that, it is perhaps best to avoid getting caught, lest they go to the prison- as something... strange seems to have found its way down there and made it home. Though, perhaps they could also deal with it...? Hm... Well, if you want to know where the key is...

anyway here's a Seam doodle to make up for the boring answer haha

concept.

they are sarcastic and dry, and not wholly against violence, but aren't demanding of it. they are not the greatest person, but they have good intentions.

Aarch is, of course, based off of Chara. but Frisk does not personally know Chara, and never did (this Frisk never did No Mercy), only heard stories and listened to the tapes in the Secret Lab. They don't know what Chara looks like beyond being.. similar Enough to them.

So Aarch vaguely looks like a mirror of Frisk, and a little bit like Flowey since Frisk knows their connection to Asriel. Aarch is not the best person, because Frisk knows that Chara wasn't- that's one of the few things they were told about Chara by Asriel. But... their intentions were good, and... Frisk is an optimist that sees the best in everyone. So Aarch is a reflection of that. Aarch is the image Frisk has in their head of Chara, of the child who they know they were both so alike and so different from. One of many children who's deaths long ago paved the way for Frisk, made manifest through their old Heart Locket (another thing of theirs, which is now Frisk's...)

This AU is really neat, and I just wanted to share a random ass mental image with you.

Frisk does end up telling their family about their adventures, however everyone just thinks they're very vivid dreams.

Haha, I've mentioned that before actually! Frisk DOES Tell people about the Dark Worlds and their adventures (or, at least, in the hypothetical 'game' for this AU it'd be a dialogue option present when talking to most characters.) Its just that most people simply assume Frisk is playing pretend or dreaming; Frisk is a very creative and imaginative kid, so that seems like the most reasonable conclusion.

It is, however, the wrong conclusion.

You have been through this once before.