What is overlooked is the angle of the bullet's trajectory. Very few people have shouting matches sitting down, unless they are unable to stand in the first place. Thus, the angle of the entry and exit wounds of the bullet that killed Jonathon should have indicated that he was shot either by someone on their knees (unlikely) or by someone who was in a seated position. The only suspect who fits that description is his wheelchair-bound mother - unless she actually has the ability to stand up on her own, something which is never implied.