That can be really frustrating for sure. Were P1 and GM used to playing with each other? I've definitely been the newcomer to an established group with a "this player loves the spotlight, the rest of us are here for socializing and the stabby parts" vibe. Though your P1 sounds like they were a bit of an asshole even apart from any habit of being the only protagonist.

(I've also been that person, either on purpose because it seemed like otherwise things constantly bogged down in planning paralysis and indecision or inadvertently when that's just how the party shook out. Sometimes it's worked out, sometimes I've had GMs or other players tell me to cool my jets.)