Paying her not to talk about it isn't a legal failing, but a moral one.
Whether doing so should be legally considered a campaign expense because it could affect an election or a personal one because it could affect a marriage or some degree of both... whether associated reporting is done correctly for technical campaign finance rules and how funds were or weren't comingled or moved and how one bills and utilizes attorney fees...
These are only questions you ask when you already know what answers you want to find.
Choosing not to vote for a politician for moral failings is totally understandable, but Trump's supporters already knew about the affair and payoff and didn't care (or didn't care enough not to support him in the circumstance they did).
I guarantee you there's close enough to zero of his supporters upset if he used their donations to pay off a porn star as opposed to run another ad or something, as long as it's something that could have helped him win.
I'm not a supporter, but I also think campaign finance laws are by and large bullshit.
It wasn't a lot of money for Trump, and whether it was paid for by him or his attorney using attorney fees and whether the attorney was acting on behalf of the campaign or him or both...
Both Trump supporters and regular people who don't support Trump collectively ask... "who gives a shit?"