He seemed really shaken by the assassination attempt, some staffer said he obsessed over it to the point of rewatching the video of it on loop, so if people keep popping off around him he might seriously break down.
Trump would step down, leaving Vance as the nominee, who would them nominate a new VP. JD then steps down, leaving JD's pick as the new nominee who chooses the new VP.
That's true, JD could show character and sound judgement by voluntarily removing himself from the top of a presidential ticket. I didn't even consider that as a possibility for some reason