Good to see you back at it. I have saved a copy and will read when I'm at home, and hopefully comment later.

~Neshomeh

I especially like Anthony's training, and how he can't quite manage to stop singing things. I also like how you make a giant purple centipede appear affectionate and almost motherly.

Nice work!

Impressive work, and good riddance to a nasty set of Sues. I love the reaction to the crown of many colours.

A couple things :

"“It’s easy for you to saw,” grumbled Tia" may need to have "say" instead of "saw", unless this is a pun.
In "Sweeney Todd screw-up", it's typically treated as just the two words "screw up"
"The owner of the second voice, good-looking boy" probably could use an "a" between the comma after "voice" and "good-looking"
"snuck" (five instances) is technically accepted within most of the United States, but sneaked is preferred, especially in works with an international audience. There are also parts of the US where it'll still get odd looks.
"“Probably”" could use a period at the end.
"But therre’s this other chick, Firalyn," should be "there's".
I think "“What is it?” came a beautiful and musical, but rather surly voice." would have a comma between "surly" and "voice", unless "but rather surly" is a restrictive modifier. If it is restrictive because surly is necessary for identifying the voice, then the comma after musical should be removed. As it is, beautiful and musical don't have a noun to clearly modify.

On the extreme stylistic nitpick side:
You've got someone who's supposed to be leading the narrator to Elwin's room brightly announcing that they've arrived, and then the narrator stating that they've gotten to the door to Elwin's room. If there's reason to clarify that there was no duplicity, this is useful, but if there is, I've missed that reasoning. There's a pretty good opportunity for a joke here based on duplicity -- for example, pointing that the room of a Sue is seldom a good place, but that the addition of a giant centipede, several Orcs, and a trio of mini-balrogs spiced up the scenery -- might get it out of the psuedorobotic tone it falls for a second there.

Likewise, there are a few places where both agents in a scene respond to each with very similar sounding tone, most notably when both say "All right". That's actually very realistic, especially if the Agents have spent much time working together, but it doesn't look great in prose. If you want a theme of individuals who are similar inside despite how different they are physically, this can be a useful subtle trick, but I dunno if that's what you're aiming for.