also, the order to have the rival killed is itself an official act (directing the military, commander in chief power) so you can't introduce evidence of that even if you could somehow find a way to prosecute the assassination
I'm convinced that the Founders were smarter than anyone currently on the bench. I always assumed that studying the Constitution gave everyone a sense of humility and history. Appears not.
But that's only true for the (disallowed) prosecution of the president, right? As far as I can see, Trump v US disallows using evidence of official acts to prove a criminal unofficial act. It does not say that the same evidence can't be used to prosecute people down the line. (Another can of worms.)