I think what is going to included in the indictment that Trump paid back Cohen through Trump Corporation and that was recorded in the Accounting records as a Legal Expense. That, if proven, could be a problem. A basic rule of reporting and taxation is that personal expenses are nondeductible for Corporations/Businesses.
So from what I have seen reported. Trump had Cohen set up a Shell Corporation not tied to Trump Corporation. No problem. Cohen paid Stormy the $130k through the Shell. Trump reimbursed Cohen but rather than pay him personally or maybe through a Trust he has, it appears that Trump Corporation paid Cohen back and that payment was recorded on Trump Corporations Accounting records as a “Legal expense” which was then used as a tax deduction.
Had Trump Corp paid Cohen and set up a Receivable/Due From Donald Trump on the books of Trump Corp when they credited cash and included terms of repayment (time, interest rate, etc) and Trump paid the Corporation back then I don’t see a legal problem.
I have read, and who knows the accuracy, that some payments were made by Trump Corp, and some by Trump using pesonal accounts. I have never seen anything on whether Trump paid back the Trump Corp for any expenditures on his behalf.
Trump could make a valid argument that the Trump Corp, which relies on his reputation and “brand”, has a major interest in keeping the Stormy Daniels story out of the public eye. Half or more of what Trump agreed to pay Cohen was for his services, as an attorney ($130,000 for the Daniels agrement, and something for a Playboy Bunny claimant, leaving about half the $400,000 something for Cohen). All this is messy, and not the way I would ever do business, but I have my doubts on whether any of it was illegal when all the facts come out.