Very good, very PLAUSIBLE theory for sure.

Only thing is they would have to kill the patsy on scene, any forensic expert worth their salt can tell if a body has been moved, it's evident in the way blood pools and drains from the body. Patsy would be dragged along on scene to watch what he is going to be blamed for then murdered.

Name one high profile shooting incident that didn't initially start out as multiple shooters, then got retracted back to "one single lone-nut" shooter. It happens every time, because there ARE MORE THAN ONE!

Another theory is that it's like all the "terrorist" plots that the FBI plans, organizes, foments, and then gets a mentally deranged patsy to execute while the FBI holds their hand. This could be the same thing, where they make one or two professionals go in with the WILLING IDIOT, or DRUGGED UP idiot, or whatever, and then once the job is done they kill the fall guy and the pro's leave the scene.