Your POV is interesting to me. I thought her testimony as to the relationship is part of the case. It doesn't make sense that there would hush money without an action that one wants to hide. If she was asking for money in order to keep this quiet, she would be blackmailing him. His machine offering her hush money requires that there is something to be hushed. Mr. Trump has always said there was no interaction and turned this into a he said/she said scenario. If he had admitted to himself and his team that there had been a relationship and he wanted her to sign a non-disclosure agreement then that is not illegal.
He created the storm here by refusing to admit what he had done intitially AND, then, he doubled down with the money offer, AND, then, he called everyone who helped him liars and her one as well. So,this is the reason that her testimony has importance from my perspective..........JMHO