I always thought that trying to use congressional proceedings as equivalent to legal precedents was very dubious. A senate “conviction” could lead to a real trial of course.
Yes. I don't see the Supreme Court or any inferior court wading into that can-o-worms.
Gerald Ford said "An impeachable offense is whatever a majority of the House of Representatives considers it to be at a given moment in history" and he's correct, as Impeachment is first and foremost a political remedy.