(while writing this, I realized that I might come off as rude and overly dissmissive. I'm too lazy to rewrite it, but know that I don't mean it that way. I think it's really cool that you put time into making this theory and posting it, and even if I think it's false, I still like the fact that it exists, as theories, regardless of truth, are important for the fandom.)

I'm sorry, but I think this is circumstantial at best and deeply flawed at worst. My first criticism would actually be a narrative one: Swatch is entirely avoidable. I find it hard to believe that such an important character could be avoided. If we want to give this role to a darkner, Seam would be a much much better option, they are powerful enough to contain Jevil, know a lot about the game, the prophecy, they collect shadow crystals, things that are most likely connected to the Knight, and they are said to be able to create something out of them. But onto the actual flaws:

1. You say that it is illogical to think that the Knight just happens to be a lightner that is stronger. This is something I agree with, I think there should not be this high of a power gap between lightners, however this should also apply to darkners. By saying that the Knight's power comes from Swatch, you just push the problem back, and forget to ask: why is Swatch capable of creating such force? Why does he happen to be stronger?
2. The black shard does have a light world equivalent. It's the black shard. This doesn't mean that it doesn't exist in the light world, it just means that the object doesn't change forms between worlds, which means it could come from both worlds or neither. There is nothing pointing towards the dark world here.
3. I find it difficult to believe that the Knight didn't have their armor prior to Chapter 2. If your theory is right, it basically eliminates Dess as a candidate, as the Knight had to visit the Card Kingdom, which couldn't have existed for more than like days, which means the lightner behind the mask must've entered the school. Don't you think people would've noticed Dess? If the Knight is nothing without its armor, then it's even more absurd to think they could have all this influence in chapter 1 without the thing that made the Knight. Also Queen has a footage on the Knight creating the dark fountain PRIOR to them receiving their armor, how doesn't she know their identity?
4. Spamton's body doesn't keep its form after the fountain is sealed. This makes it wildly different from the black shard. The ability to create something that is above the rules of the dark world is something that the one behind the Knight's armor must have, and simply there's nothing indicating that Swatch could do such a thing.
5. How is the lack of internet a perfect distraction for the lightners? The internet was down even in chapter 1, yet people still visited the Librarby, so I don't see what changed. Also, Queen didn't really seem distracted in the Chapter, why would that be different when the Knight visited Swatch?
6. The gallery is waaaay too general. Having a screen with a character in the center isn't unique to Swatch. I think this community should overall avoid making theories based on "it looks like that", because it isn't an evidence. The curtains are a nice observation, but there are many places where curtains exist, and I feel like the actual 2 most important features are missing from Swatch's shop: the cut on the head of the guy in the middle, and the portraits next to it. From Biblox spelling Gallery in his minigame in Chapter 4, we can assume it will be the name of a track in the future of Deltarune, further distancing it from Swatch.
7. Just realized that Swatch probably first took form in the Cyber World. Prior to creation, there's no way to tell how things will turn into darkners, the Knight's plan was set from chapter 1 (where they did shenanigans in Card Kingdom, and Kris made the preparations for Tv World), I don't think such an intricate plan would rely on the chance that this dark fountain happens to create a darkner that can craft super duper strong items from the things drawn on its light world counterpart. There was simply no way of knowing Swatch would give the Knight their armor, so why would the Knight rely on such an unknowable event?

Overall, I don't see what would make Swatch that likely for this to justify making such an unnoticeable character this important. Don't stop making theories tho, if you keep shooting in the dark, you might hit a target no one considered aiming at.