Those questions are crap, and a subpoena to answer them is real crap. It’s all a smear campaign.
Mueller has already decided Trump is guilty. They’re just going thru the motions to pretend the process is “legal”. They’ll subpoena him, legal or not. It then doesn’t matter if Trump honors the subpoena. If he doesn’t, they’ll charge him. If he does, then his answers don’t matter. He’s already guilty. Coup complete.