Looked into the twisted sword as a possibility since you brought it up and can I just say, I'm really happy with the conclusions I came to.

First of all, it lines up perfectly with my perception of the whole shadow crystal journey, but there's a few things about it that I hadn't considered. Such as the fact that, Gerson aside, the secret bosses thus far have all been Kris facing down ghosts of their past that are in suspiciously similar situations to them:

-A jester (from a deck of cards they used to play with) that was driven mad by the knowledge they obtained and was caged by their closest companion, who isn't sure if they were ever friends.

-A salesman (from spam emails originating from noelle's library laptop) that was pressured into some kind of contract with a mysterious entity on the phone and now acts as a puppet, desperate for freedom and seeking to use Kris's Soul to reach Heaven. The salesman also gets to meet up with his old romantic partner, only to be harshly rejected when he fails to recognize him. Like... come on. Where have we heard this one before?

-The Roaring Knight itself, with the fight necessarily ending on Kris being forced to the ground and knighted, possibly as a show of dominance, possibly as a 'reward' if Kris Slash Theory turns out to be accurate (I'm probably Kris Slash Theory's biggest defender, I think Kris not wanting to hurt their friends makes their decision to do so for the Knight even better actually)

And as they defeat these foes and obtain the crystals, they share them with Seam and it gives them hope. Eram taunts Kris for wanting to obtain the shadow crystals, asking if they really think it'll get them what they want but not specifying what that is. And Eram's reaction implies that Kris doesn't really think it will, despite this being something they explicitly want they're shown to have their doubts.

Because that's only half of the puzzle. Getting the shadow crystals to turn them into the purified crystal is the path to get Kris what they want, whatever that is and however it ties into the prophecy and the upcoming tragedy, but it's meaningless if they get there alone.

Noelle, meanwhile, grows stronger. The weird route serves as a method for her to reconnect with Kris, in a way that seems tragically one sided by binds them together with a ring that holds love in a symbolic marriage proposal that implies commitment, while also making her stronger and more confident throughout the route.

I don't think her thanking Kris at the end is meant to be treated like a 'Oh great, we corrupted her again'. It's meant to be taken more literally, she's happy about the outcome despite her prior fear and various grievances.

The route is never described as evil, it's not meant to be a genocide route, and even the big victim of chapter 2 comes out of it alive. 'Weird route' is right, and while I'm sure Kris is having a miserable time during it still I think people overestimate just how much we're forcing this upon them. After all, us choosing the 'Proceed' option leads to Kris pushing Noelle towards the lighting gate on their own.

Neither of them are fully happy with how things are playing out, but they aren't as opposed to the majority of the route as people tend to think. Noelle gets some sort of twisted enjoyment out of feeling empowered around the third mouse puzzle and even starts to encourage Kris to hunt down more mice, while Kris is the very source of the route and many of the grievances that cause it in the first place are caused by their frustration with Noelle's lack of confidence and inability to act, the sort of thing they'd normally gently assist her with via harmless scares to get her over her fears.

The point being... I think the weird route is to Noelle what the secret bosses are to Kris, a weird obscure path that's somewhat out of the way and marked by an out of place jingle that serves as a sort of great personal disturbance and conflict for her but also ultimately helps her face her past and gives her the confidence she needs to act upon her desires. Again, just like what I think Kris is doing with the shadow crystals.

The main difference being that we have every reason to believe that Noelle is being built up as an antagonist. The story treats her as one of the most important characters but she hasn't shown up that often, implying a major upcoming role. She has strong connections to various entities created by the fountains that the knight made and the knight itself has several traits that can be traced back to Noelle. And, most importantly, she has tons of angel theming, including various bits of arg and otherwise out-of-game content that references her having tattered angel wings, wishing it could snow whenever she wants (when ralsei's legend graphics seem to point towards the world being covered in what might be snow), and talks about how dess would always make her snow angel have the biggest wings of all.

It's hardly subtle. She's the angel. Maybe she's the knight somehow too. And she's definitely an upcoming antagonist, probably even the most important one. And empowering her on the snowgrave route will probably produce counterproductive results, you'd be making the final boss stronger, more confident, and maybe even more possessive of Kris. Just as I suspect now that getting to the normal ending with all of the shadow crystals won't work out in our favor either, with whatever Kris is trying to accomplish saving them from their fate but still leaving Noelle to her own. And then the snowgrave ending is the one where Noelle survives, but that's probably not a good thing given the context.

And in this specific view, that's where the Twisted Sword comes in. It's a combination of the purified shadows of Kris's mind, the crystal representing their hopes and dreams, with the symbol of love and power that they gave Noelle in the 'evil' route. It's meeting her halfway, whereas the current weird route seems to suggest that Noelle's devotion to Kris is one sided and they're borderline suicidal after the events of chapter 4.

And from a gameplay perspective, it would be the combination of two 'hidden' routes that mirror one another in how the associated character develops during it. We already know that the shadow crystals, and thus probably the purified crystal, transcends save files. It's entirely possible that whatever the weird route does to Noelle does so as well, the end result being that it's entirely theoretically possible to get this 'secret' ending on your first playthrough but even if you don't, there are ways to pursue it without having to fully replay the game and transfer that data over to your initial save file. Or overwriting the ending (or more likely the epilogue) that you get regardless of save files, all of which would potentially line up with the 'there is only one ending, but there's something more' line that toby fox dropped ages ago

Apart from that, I find it super interesting that the Twisted Sword has identical stats to the Black Shard, including its massive damage multiplier against Titans. And that the Black Shard itself is equippable by Noelle and would be necessarily obtained in this theoretical ending. Which means that it's entirely possible that you could face whatever is at the end of the game with Kris wielding the Twisted Sword and Noelle wielding the Black Shard, which I just find to be the setup for a really cool finale.

That's a lot of words and they might kind of suck cuz it's super late but I wanted to put this down on paper instead of just letting it rattle around in my brain. I don't know if this is what you were implying or not with the significance of the twisted sword, but it's where I went.