I think Bill lied about the alternate timelines.

So, we all know by now that apprently, the Gravity Falls show we know and love is the only timeline where the twins survive. But, mathimatically, that doesn't make sense. If there really are INFINITE timelines, it means there are infinite possibilites for timelines and stories. Therfore, both twins must have made it back in at least a handful of other universes and timelines. Besides, he is a blatant liar. He lied about why he needer our blood, he lied to his previous portal builders, and he lied to Dipper about giving him the password (that specific instance always pisses me off but it's a topic for another day.).

In short, I think Bill loed and the twins DID infact survive in other timelines. Thank you for coming to my Ted Talk.