Does completing the secret challenge with the music-synced transformation give you access to the ball in the menu? I can't tell if it did, seeing as I already activated the ball through brute force...
If not, is there anything else in the menu besides ? Just wondering if I should keep searching or not!

Hey Nifflas, could I take a look at the editor source? Pretty please? No promises, but I would love to try and improve the interface.

EDIT: whoops, didn't even bother opening the zip before I posted that.

Does buying Lu's crystals activate The Insanity (quest)? Or is that related to the Utopioca version of Lu? Or something else entirely?

Yeah, I got to 3 keys and then read this thread (after scratching my head for a while). 

There are some spiders in Knytt that move like that. They are not enemies, though, if I remember correctly, but this means they are probably easy to implement.

Now remember: the whole of Knytt was hard-coded, and there was only a minimal amount of intelligent behavior - as I recall (having looked extensively at the Knytt source), those spiders were on a fixed path, defined in the source. If Nifflas wanted to make a movement like that work in an engine which supported level-editing, and thus arbitrary surface shapes, he'd need to do a lot more work.
It would be far from impossible, though, but still a lot of work.

Luscious Nifflas...
erm, that may have been a bit of a stretch

That's funny, I use the same loadout as you. Straight.
Except that for the longest time I was trying to craft the Kritz and constantly getting more Quickfixes... 

#2. I'm a sucker for mystery and finding things myself