Would proof of the existence of Bilbo change your view about Rambo?
If gods with great supernatural power turned up, but didn’t claim to be “God”, that’s not evidence that the (non-appearing) “God” exists. However, I would be convinced that other planes existed for such dieties to hide in, which I currently do not believe. So that would increase the probablity slightly. From zero belief to zero belief-with-a tiny-bit-less-certainty.
Though if the other gods turned out to be a contradictory pantheon, though, such as the norse pantheon, I would believe in God even less; this universe was only created once, after all.
If a dude who claimed to be God himself turned up, I’d run for cover. He is generally portrayed as being tempermental and judgemental, with no fondness for those who did not believe without reason, and so my best bet would be to stay as far from him as possible - if he let me.
No; the existence of an evil liar doesn’t imply the existence of a dominating smiter. Heck, even if Satan said God existed, why should I believe him? He’s an evil liar!
Remember, this thread is about what can be known.