The reason Trump and his defenders aren’t mounting any kind of serious defense to the impeachment accustaions is even simpler than “There isn’t one”. It’s the simplest reason of all:
He doesn’t need one.
The Senate outcome is preordained: they’re going to vote to acquit. Trump knows he doesn’t need to defend himself, at all. Instead, they’re using the media attention to stage another episode of Republican Grievance Theater for the rubes.
Hand-wringing over the incompetence of the defenses misses the point. The defenses don’t need to be legally competent; they only need to keep the GOP base happy.