One thing I'm left confused about a Temmie <> Flowey and Asriel <> Monster Kid swap is that, well, how did that even happen? It works well with Flowey cause he's just a flower with Asriel's dust and essence infused inside it alongside injected determination by Alphys... But how would that work for Underswap's case? Random dust getting on a random Temmie and Undyne doing experiments on it? Makes no sense..? (at least if you're trying to stick as close to canon as possible)

Tell me if I'm wrong here tho.

You'd need to do something entirely different for something like that to work. Wasn't the Temmie <> Flowey and Asriel <> Monster Kid swap a joke suggestion anyways? (idk about swap lore)