Standard advice is to show, not tell. But if you’re already doing a lot of visions and such, an extended third-act flashback may end up being just too much showing — interfering with the present-tense storytelling.
So maybe the answer is to tell, not show.
Is there a character who could be really compelling telling the truth about what really happened? There are some great movie moments that are just two people talking about the past, and may you could make one.