Honestly I don’t think they could have anything that would do any lasting damage to Trump. He has already shown that he’s above the law and his supporters literally don’t care. What they could threaten him with though is cutting off funding or exposing his actual net worth. Either one of those could really hurt him. All that said I don’t think they even need to go that far, it’s obvious Putin controls Trump and gives him his marching orders, so all it would take is a call from Putin.
Last one is possible if unlikely. Probably won’t know much more till the blackbox analysis is done. I am somewhat reminded of a rather famous crash that was caused by a pilot inviting his buddy who had some flight experience but not in that model of plane into the cockpit and had him do the landing. The buddy screwed up a couple of critical settings and the actual pilot didn’t notice until it was way too late to prevent the crash. Not that I’m suggesting anything similar happened in this case, it was just a very similar style of crash.