Maybe even similar to Harry Potter walking back through Hogwarts after the big battle.
Maybe change the colour correcting to Red, Mustafar.
I doubt it but maybe there’s a way to splice the Mustafar raid with the Kylo v. Vader fan film to make it seem like while he’s fighting that giant group he’s actually just envisioning taking down Vader?
Maybe Vader is a projection of the Wayfinder? When Ren defeats him, he opens the casket and removes it?
There’s a post-credit scene in the fan film where Kylo is about to remove his helmet, so that would help with maintaining continuity.