What should a state do when the most prolific invader in the world (i.e. NATO) starts amassing troops and weapons on its borders (or otherwise in position to attack, like with Iran), especially after a coup against a neighbouring state?

Maybe not invade a neighboring country yourself, and then keep investing material and manpower after the invasion has stalled for no discernible gain.

The one that hasn’t been substantiated, one which the NATO-dominated UN concluded to there being no evidence for, one which your masters have long since stopped pretending to believe to have happened?

What the UN conclude was that while there are reeducation camps and systemic discrimination, there is not sufficient evidence to say its a genocide. Which is like "We can prove that A killed B, but we can't prove that it's murder".

And no, Adrian Zenz’ word is not proof.

Funnily enough, it's only ever the deniers who bring up this guy.

Oh, hey, it’s the same people that sold you on Nayirah’s testimony. Do you believe that one, as well, since Amnesty said it was true?

That's a quite deliberate misconstruction of the facts. Amnesty International initially promoted the story, then they issued a retraction once they found that it lacked supporting evidence. There's no evidence of malpractice on their part.