But if his theft of the documents was legal, then what’s to say he didn’t secretly declassify the documents without filing the correct paperwork, which, as President, he was legally allowed to do?

If the Courts rule he was above the law, it gets screwy.

But that’s all academic, because there’s no way the Court is going to rule that Presidents have blanket immunity from prosecution if they aren’t removed from office by a Senate conviction. There’s literally nothing in the Constitution remotely suggesting that. In fact, it specifically says that criminal conviction is an entirely separate process from political impeachment, and that an officer can be charged criminally separately from an impeachment.

The argument is so absurd his lawyers should be censured for bringing it to the court.