I don't see how this is not easy to see.
Trump's defense now, at least from the advice of his lawyers, is to claim the payment had nothing to do with the election and instead, was done to prevent Melania from finding out.
His lawyers have admitted to the payment and apparently it was classified as a "legal expense". So, we got a misdemeanor misclassification of a business expense. I see a plea deal in the works. But, if the payment was done to prevent the knowledge of the affair from coming out during the election it becomes a felony, falsely recording legal expenses to avoid a crime, namely infringing a New York State election law.
And that is where Melania comes in to play. Did she know about the affair prior to it becoming public? That makes the hide from Melania defense a lie. Using your words, "one of many", if Melania knows that Trump cheats he wouldn't have to pay money to hide them, again, makes his defense a lie. At the very least, Melania should be deposed if this becomes their defense. Although it does appear Trump is hesitating.