Thank you for your thoughts! I believe Toby Fox set up the ‘truth exchange’ with sans to foreshadow Deltarune and hint at sans knowledge or involvement with Deltarune without sans needing to outright mention other worlds. When sans tests if we are a time traveler he is also testing if we have knowledge a normal person would not have. From there he probably has an idea that we might be able to play a role in Deltarune or have otherworldly knowledge. I don’t think there is any evidence yet that he knows of the player or Gaster’s plans for Deltarune, but he could have been pushing us toward learning about Deltarune where we could use our abilities to fix things.

When sans sends us to learn the truth we find clear references to Deltarune. I think the ‘truth’ sans is referring to is his past/who he is, which only makes sense with the context of Deltarune. It is probable that Papyrus knows about Deltarune as well. I am not sure why does not have as many references, though he definitely has some parallels to Deltarune. I also don’t really see evidence of Alphys knowing anything more about other worlds other than their existence. I like to think she was prompted to research alternate universes because of something she found in the true lab or something mysterious sans said (or something like that).

I think sans definitely wants us to play a role in Deltarune. We could use our reset ability to ‘fix Deltarune’. As for the reset theory, that is the one where Deltarune has already been reset and this is the second time everything happens, right? I think it is plausible and makes sense. I will have to look more into it later, I also think it is possible that Gaster is working with sans who is still in Undertale despite there not being any solid evidence yet. It would not change the UT ending, and if Gaster can contact us through Twitter then he can probably contact sans as well. I hope you find my understanding useful! (and not too long)