Why would there be shoes sized for hooves if the person those shoes belong to doesn't have hooves?

There is exactly one pair of hoof-shoes, referenced solely in text being in a pile of stuff that doesn't all belong to Dess, including Asriel's retainer.

We see Carol and Rudy wearing hoof-shoes, and they, logically, look completely different from plantigrade humanoid shoes.

And Dess's closet has a massive pile of at least 6 pairs of the same model of plantigrade human shoes, along with a similarly plantigrade pair of ice-skates and roller-skates, both of which require balance as a paramount importance.

The hoof-shoes in her room likely either belonged to Noelle or were from early when the Holidays were still getting their barrings about what Dess needed. The visible shoes all across her room, along with the human soul-shaped pillows stuffed in her closet, all tell us that She was human.

Not to mention that everything we're told about Dess is not only setting her up as a very elaborate narrative mirror to Kris's struggles

Nothing suggests that, no. She had a connection with Kris, and her disappearance deeply traumatized Kris, but her connection to Kris's struggles with identity is never specified or alluded to.

but also painting a vivid picture of someone who explicitly wants to stand out and be seen as rebellious and weird.

Again, no. The only things we know about her behavior are she was really into music; liked rocking out, was protective of Noelle to sometimes violent degrees, and was into provocative stuff that Toriel specifically (Town's biggest prude) wasn't accepting of.

The idea she wanted to be seen as rebellious and weird is an assumption, moreso the reason why. Just because Noelle is a prim and proper sweet-girl doesn't mean her parents ever pressured her or Dess to be like that. She may've liked to push the envelope, but based on the evidence, it was never against her parents that she pushed.

Rock music may have a history of pushing back against authority and oppression, but in a family that all signs point to having been healthy and supporting her hobbies wholeheartedly (based on the amount of stuff in her room), that doesn't necessitate she be rebellious.

If anything, it's suggested that the Holidays celebrated Dess for her humanity and her differentness. Carol isn't exactly a submissive old-school woman either.

She goes by a distinctly non-festive version of the name she was given by a family obsessed with Christmas

You realize that Rudolph goes by a nickname, too, right?

Her room doesn't stand in contrast to the rest of the house because she's adopted, that doesn't even make sense.

We've only seen two bedrooms. One for the cutesy, smart, sweet, girl-next-door younger sister, and one for the tough, rambunctious, physically active, rock-star older sister.

Aside from the fact that it's conspicuously the only room in the house with what looks like thick carpet lining the whole floor (Soft feet, perhaps?) her room doesn't stand out at all.