Doomed timelines and loops can only happen after they've entered the session right? I keep seeing headcanons about pre-session doomed timelines and I'm never sure how to correct them, can you help please?

Pre-session doomed timelines can happen. There was a doomed timeline where Karkat didn’t run the Double Mobius Reacharound virus and also Gamzee flipped out before everyone entered the game and he killed all 11 trolls.