All of those questions are matters of prosecutorial discretion, with huge latitude.
But given your experience & observations, what are your best guesstimates on such?
He doesn’t have to get a deal, POTUS can merely have it ‘leaked’ he is cooperating! He will spill in order to get protective custody in hopes of avoiding Arkancide. Simply need to have the authorities take him in for questioning, keep him several hours and turn him loose.
Perception is reality, the more he denies the more he is implicated. He chose to swim in the swamp.