Because if he gets a mistrial, the State gets to start all over and do it again. Maybe the get a hangin judge next time. Maybe the get to coach their witnesses a little better next time. Maybe some witnesses change their story or get killed before next time. The State is evil. The State is trying to kill Kyle Rittenhouse. It's the same power and ideology that's trying to kill people with the vaxx.
Very unlikley. The State could appeal that ruling to the appelate court and have the Judge overruled. Mistrial w/prejudice is for outright prosecutor misconduct like a fake witness or invented evidence, not these typical games this prosecutor is playing. Kyle killed people. Had he been a cop it would've considered "Justified" but Kyle isn't in the elite protected class like cops are with their qualified immunity. He's just a pissant/peasant like us anons. A cog in their machine to be used and spit out.