I got no clue how this fight will end, but for the situation with Deku and his arms, I got a couple of ideas.
It's unlikely that Eri will be of any use, I feel like her appearance a couple chapters ago was just so Horikoshi could make that clear, since she doesn't have enough energy stored up to help him. It's possible that maybe Deku could sit by for awhile until she's ready to go, then she could restore his arms, considering how she was able to restore Mirio's quirk several months after it was lost.
That said I'm leaning more towards Deku just getting mechanical replacement arms. They could even be based on the specs and capabilities of the suit that All Might used against AfO. And as for Deku's future as a hero, that'll depend on whether or not he still has OfA which honestly it's hard to say. All the vestiges, including the first, are all gone now so that could mean OfA is gone with it, unless perhaps Deku is back to how he was before, only having the super strength and no other special ability.
But considering how Horikoshi initially wanted the story of MHA to be about a quirkless Deku becoming the number one hero, I'm leaning more towards him coming full circle and finishing the series quirkless, but still a hero and using armor based on what All Might used. Honestly though I'm not really a fan of that idea, since if the title of number one can be decided by whoever has the shiniest suit of armor, then that would really make it a super trivial thing, that hypothetically anyone could achieve.
It's also possible the Chisaki could factor into what will happen with Deku. Overhaul is an incredibly powerful quirk, and it could fix Deku right up. Chisaki still possesses it, he just can't use it cause he ain't got any arms. What they could do though is have Monoma copy his quirk and then use it to assist Deku and fix him back up. I do think this is a good possibility, since Deku and Chisaki did see eye to eye earlier in the series, with Chisaki agreeing to cooperate with him so that he could see his old boss again.