1. It took the other romances to get there, sure, but then it's all erased for the final bit. No one else but Dave remembers it happening, which is the problem. Perhaps canon isn't the right word for the situation, but it has the same effect for everyone that isn't Dave. That's the central issue I have with it.
2. It wasn't a perfect suggestion, I was just throwing out potential alternatives. It's your story and your sequel to write, I was sure you already had plans for things. Saying the medicine doesn't work because of what Roswell does on day 4, sure, but... why? That kinda feels like a "just because" situation.
3. No comments on this bit, you have plans/desire to adjust things so that's good there.
As for the mansion, sure, I understand that. It's just a real bad feeling that, like that reddit comment said, all the character growth and relationship development just disappears once the experiment concludes. I don't think something like that could ever feel good to the reader. Your time investment doesn't matter except for saving Roswell, and even then both Roswell and Dave are mentally worse for the experience. (Besides the whole, Roswell being actually alive thing, which yeah is important). Sure, you can argue that it does still matter for you, personally, since it did happen for the reader but... I just don't know. I don't know what else to say at this point.