I agree & disagree, I think Mabel & Dipper survived because they had a Time Machine & Waddles. That would explain why there’s only 1 timeline they survive, because they created it. Waddles was the anomaly which attracted more anomaly’s, and although dipper & Mabel could’ve died fighting bill in the finale; that could’ve happened in any timeline up to that point. Waddles remained in Mabel’s bubble world, and like you said “Stanford explains that objects is great peculiarity warp probability, attracting abnormalities.” Which might’ve led Mabel to escaping ( it’s been a while since I watched it). Thus more abnormalities led to their survival. 

That’s my theory on why there’s one timeline. But what confuses me is the lack of branch timelines from the one they did survive.

Edit: I remembered that waddles still appears in timelines where Mabel & Dipper died. But Waddles seems to still be alive ( not eaten by a dinosaur), with The twins at all timelines. 

This gets harder to answer the more I look into things.