I'm fully on-board with the "Arthur dies and you play the last mission and free-roam as Marston" idea.
However, I don't think it's going to be Marston to kill him. Personally, I just can't see it.
The whole Dutch idea seems solid to me as he's a pretty well established bad guy type.
I thought I saw this is taking place ten or twelve years BEFORE the first game so I don't think the events of RDR2 will lead right up to the beginning of the first game. Unless there's a time jump.