However, perhaps it can faithfully restore Will's brain-state from recordings of Will in the minds of humanity anyway, if that's what humanity would want. Alternatively Will is revived in ancestor simulations done by the FAI or any other FAI that is curious about humanity's history around the time right before its singularity.

I am reasonably confident that no such process can produce an entity that I would identify as myself. Being reconstructed from other peoples' memories means losing the memories of all inner thoughts, all times spent alone, and all times spent with people who have died or forgotten the occasion. That's too much lost for any sort of continuity of consciousness.