If we accept all of this as true (and how you would prove in a court of law that it is remains a mystery to me), it is very difficult to see the legal issue here. That payment to Pecker was apparently never made so this is, at worst, the contemplating of a rather minor, and very difficult to prove, campaign finance crime, which never actually took place.
Yes, it would mean that Trump has told several lies about this episode, but the sad reality is that not even one current Trump supporter would stop backing him over such a set of facts, even if they were somehow proven. This type of shady stuff was already long ago baked into the Trump cake, and McDougal is so attractive and so supportive of him to this day that the story might even help him with some people.
There is however another scenario, which has been speculated by some, including Brian Williams and Seth Abramson, where we all have the wrong David and that the “real” David to which Trump and Cohen are referring to is the fictitious “David Dennison” who was apparently created to pay off Stormy Daniels. At first glance, there are a lot of reasons why this possibility is enticing.