I have a background with federal political campaigns about 20 years ago. I don't think any law or regulation has changed regarding in-kind contributions or candidate-contributions since then, so I'll share my opinion on this matter.
It's not a violation.
If I ran for federal office and I decided I needed to buy a car so I'd have reliable transportation, I could do it two ways:
1. I buy it myself with my own funds.
2. I use campaign funds to buy it.
If I paid with my own money and intended to keep it after the campaign, as it's a DURABLE item, then it's not a contribution to the campaign.
If I used campaign fund to buy it, I can drive it all during the campaign, and at the end of the campaign I can buy it back from the campaign if I wanted to keep it, paying fair market value. Or, I can let the campaign sell it to someone else and use that income as settling the debt of the campaign. But, the other person buying cannot pay more than the donation limit of $2700 as it's an in-kind contribution (for a material asset).
However, if I let a friend buy me the new car, or someone gifts it to me, and all along I knew I was going to keep the car after the campaign as a DURABLE item, it's not a contribution to the campaign. But I'd probably have the report it on my income taxes.
The key to me with Trump is that his reputation has value and any allegation against him from these women would hurt his reputation beyond the campaign, and therefore it's DURABLE. Candidate or third-party funded durable items are not campaign contributions. Now I'm talking about a car here, it could be a computer, or a nice suit, but it cannot be direct election-related things like tv ads, yard signs, consultant fees, etc.
The FEC has a list of direct election-related items. "Paying off mistresses" is not on the list.
So I see that Trump had a third-party handle these women financially, but it was to protect Trumps durable reputation, something that's outside of the campaign as he has huge financial interests before the campaign and will have them after he leaves office. And as it's not a campaign related expense, none of the in-kind restrictions apply.