The whole problem with this angle for Mueller is that Trump is a big enough celebrity outside of politics that he can legitimately claim that the payments had nothing to do with the election and everything to do with protecting the Trump name and brand. The FEC may beg to differ, but SCOTUS will not; in fact I'd wager good money that in this instance, SCOTUS would label the payments as 1st Amendment free speech. And no, that isn't a stretch at all. But it would be solely because of who Trump is outside of politics.