Exactly what evidence would be required to convince you?
Fairly comprehensive, and direct.
You've punted on the comprehensive part. You admit you can't provide it. You make the entire decision on one bit of evidence and then disregard all the rest with the hope that it will somehow just work out. And you don't have direct evidence. You have a default conclusion you draw when things don't work out exactly the way you think they should. You've relied on a mystery writer to give you pointers on reasoning.
A death bed confession. Footage of studio, A government acknowledgement? What would work for you?
None of that, really. That's because all the evidence still has to be explained. Faced with, say, a deathbed confession it's still more parsimonious to belief the confession was false unless all the other evidence can be explained by it. You seem to be insisting on a smoking gun. That's not how it works.