And more importantly, that's not what Trump was indicted for. Indicting him for misusing campaign funds would be straight-forward, if they could prove it.

So the fact that they indicted him for something else seems to show that they couldn't prove he misused campaign funds. They actually indicted him for recording a payment to his lawyer as "legal expenses", because it was allegedly used to pay for Stormy Daniels silence.

What I don't understand is why this is different from the many other cases when people and companies pay to settle claims in exchange for a non-disclosure agreement. Those are usually considered legal expenses.