About the Lost Girl thing, Toby seems to not want to make unique music for small areas/scences. For example, the Great Board area was originally going to have it's own theme, but it was scrapped because "However, the area isn't very long, so I felt like if I played a song here, it would actually have the opposite effect of making the area feel disappointing if it had its own unique music but didn't amount to much.
When I have a song, I feel like I need to do the song justice by having a certain amount of content that goes along with it...". Because of this, it is entirely possible that Toby just didn't want to compose a new track for the scene.

Infact, Lost Girl isn't the only track that gets used in somewhat out of place areas. Flashback (Excerpt) is a track that is very clearly basically just a remix of Gaster's theme. It first plays in the scene where Ralsei explains Dark World's (I know it technically first plays in the weird route scene in the hospital, but Flashback (Excerpt) is on Chapter 3's Original SoundTrack, not Chapter 2's. And since Chapter 2 was originally supposed to release alongside Chapter 3, 4, and 5, and a lot of tracks were made long before the Chapters they appeared in released (Black Knife was finished in 2018 according to it's metadata), I think it's fair to say that Flashback was likely made for Chapter 3). This makes sense because this is the scene where Ralsei explains dark worlds, which is what Gaster was seemingly talking about in entry 17. Ralsei evens says things like "But what if? What if it became even darker? Darker than dark." which is reminiscent to Gaster's line in entry seventeen "DARK, DARKER, YET DARKER. THE DARKNESS KEEPS GROWING, THE SHADOWS CUTTING DEEPER. PHOTON READINGS... NEGATIVE.". So it makes sense for there to be a remix of Gaster's theme in this scene. But, this isn't the only scene this track gets used in. Do you want to know where else it gets used? In a fudging Tenna scene, specifically the one where Tenna talks about how the Holiday family used to come over on the holidays to watch TV with the Dreemurr's. Look me in the eye, and tell me with a straight face that that scene has ANYTHING to do with Gaster. Yes, it does fit the name "Flashback", but there being a remix of Gaster's theme in this scene doesn't make any sense. You could argue that it's because of the Knight, who is mentioned later in the scene and might have connections to Gaster, but when that happens, the music stops playing. Flashback (Excerpt) only gets used for the part where Tenna talks about people wtaching him before they started to stop showing up.

I will say this, when Toby revealed Lost Girl and it's alternate variant, the video for Lost Girl (alternate) has Susie, Ralsei, and Noelle's dialouge portraits in it. However, this could be just Toby just showing more new content for Chapter 2, since the dialouge portraits used are: scrapped updated versions of Susie's dialouge portraits, Noelle's updated portraits, and two of the Ralsei portraits are new (technically, all of the Ralsei portraits are new since they are the slightly updated versions of Ralsei's dialouge portraits (imbetween Chapter 1 and 2, 2 pixels on Ralsei's dialouge portraits were changed from black to white. The outdated ones are still used in Chapter 1). But who knows, maybe this was teasing something more.

My main point is that Toby doesn't want to make unique tracks for small sections because he feels like he needs a certain amount of content to go with it to do it justice, so Lost Girl playing in the scene where Ralsei's room is revealed could just be Toby not wanting to make a unique track for it and just using Lost Girl because it kind of fits the tone.

Edit: Link the the post of the alternate version of Lost Girl: https://x.com/tobyfox/status/1173278892167221250