Hindenburg specialise in doing this. They know the endgame better than anyone. You can guarantee they have a plan which will result in them getting paid off if their short thesis is correct.

My naive understanding: In a short you borrow then sell immediately. If the company disappears you keep the money from the sale (which you already made) and never have to return what you borrowed.

I agree with you 99.8%. I think there would be a clause in any short contract where if you were unable to provide the stock at the end then you would be liable to pay the cash equivalent. If the stock had "gone to the moon" you would be bankrupt but if the stock had gone to zero then your payment would be zero (or maybe miniscule)

There is no such clause, and yes you would have to figure out how to obtain the delisted stock from other holders (who are generally not friendly towards you).