For them to go forward to enforce the NDA, Trump basically has to admit to the affair. There is no other way around that, from where I am sitting.
Either that, or they have to argue that Trump routinely pays everyone he meets $130K and puts them under NDA. Which, while it WOULD help explain how he has gone bankrupt so many times, its not even remotely plausible.
Then, once Trump admits the affair…AND they try to enforce the NDA, it all comes back to, where did the $130K come from? I.E. Money Laundering and campaign finance fraud.
And given that Mueller is currently investigating him and his entourage on such charges, that is an incredibly stupid path to go down for Trump right now.