How do doomed timelines that start pre-entry work? would it be feasible for a doomed timeline to start just by all players agreeing to never play SBURB and living their lives out outside of the game normally (as normal as it oculd before the world ends due to meteors)

As long as it’s something they’d decide, yes. There’s arguably not an infinite amount of Timelines that can happen, some things are predestined just by the way you are. Speaking of which, didn’t Vriska at one point mention about a hypothetical Doomed John that died by getting hit by his Dad’s Car at some point pre-entry as an example of Doomed timeline? Or did I make that up?