He doesn’t need to go under oath. Did you listen to the former FEC chairman? You can spend money to help your campaign without it being a campaign expenditure. It all depends on whether there is even a chance the purchase could be for other reasons. Oh, and it helps if you spend you own money - which Trump did in this case. He could do it in cash, by check, by bitcoin, through a corporation, through a lawyer. It doesnt matter.
It has been documented that this has been a topic with Stormy for years prior to the election.
I know Cohen said what he said - it doesn’t matter. He can testify under oath that the sky is red. Does it make the sky red?
Listen to the recording again, unless you never listened to it the first time.