If a god tier player from a doomed timeline goes back to the beginning of a session to try and prevent what caused the timeline to be doomed, can they survive being from a doomed timeline if they’re god tier?

Nope, they’re marked for death unless they get ‘resurrected’ somehow, or unless there are heavy Juju Shenanigans. Given how Paradox Space operates, they’ll likely die preventing the Doom of the Timeline and as such count as Heroic.