The simplest conspiracy is to recruit a single shooter from crazytown and just make sure he succeeds by intentionally degrading the response. That has the most plausible deniability. And it conveniently fits into gun control narratives "gotta keep the crazies away from the guns!"

Why on earth would you make it look like a complex op, the sort of thing a random crazy person wouldn't do? If Trump hadn't turned his head at just the right moment he'd be dead. So you only needed one shooter anyway.