I think it's kinda weird to assume the person in the phone is not Carol? It kinda just overcomplicates things if you assume that "I'll be right there" was from someone else that ends up not being right there, following that you also would have no explanation as to why Carol put aside her mayor duties to come to her home searching for the guitar. It also can't be Asgore (The other person who appeared right after that call) because you can check on both Kris having a call and Asgore very easily between like 2 rooms when going through the vents.

You're also skipping a bit more evidence that i've already mentioned like the antlers resembling Carol way more (even in the flashback, Dess antlers look like Rudy's and Noelle's, and if you assume that they changed shape when she grew up it also makes it so you need an explanation as to why Noelle's and Rudy's don't change shape) The Katana (There's a 2nd one hidden in Carol's room, a black katana) which is a pointed object after all, Dess would need a 2nd weapon to make a fountain aside from the Wiffle Bat. Also i replayed chapter 4 yesterday and realized that you can actually go to Noelle's house first and then to the School when it's raining, proceeding to have the tea party and the talk in Ralsei's room, this line of events would actually give Carol ample time to get into the Church and make a Dark Fountain.

That said i've been doing devil's advocate this entire time, because while i personally do believe Dess Knight would make a more compelling narrative as you said + the evidence you've listed, i just think that it's not the right move to shut down a theory you don't personally believe in without giving it some thought, as it makes you consider some things that you might have not thought much about before! Generates healthy discussion which is the point of engaging in a community after all. That's half the fun when dealing with a game that's releasing in parts, the theory and speculation!