Would anyone really good at SpaG be interested, please?

I'm... really sorry to say this, but this whole mission was just riddled with errors. A lot of them. Distractingly so, to the point where I was noticing them more than I was the story.

The songs had also helped him navigate: HQ’s dimension-bending architecture required everyone to distract themselves somehow in order to get to where they needed to be.

Colon needs to be a semicolon.

The Response Centre was rather empty, with no other furniture besides another bed next to a door in the back of the room, two chairs, and another door on the left side, although a kitchen was also located besides this second door.

Besides should be beside.

“Let me enter, please. I’ve been walking around this damn place for hours, in fact ever since this flower with a bowler hat sent me out after I dropped into his office, and even before this my day wasn’t exactly wonderful, so I really need the rest.”

Run-on sentence. The comma after 'hours' should probably be changed to an em dash, and there needs to be a comma after 'in fact'.

Anyway, we’re somewhere in the labyrinth known as PPC HQ--that’s short for Protectors of the Plot Continuum.

You were using em dashes earlier in the mission, so the double hyphen should be replaced.

Richard stood up at his turn, clutching his side.

Are they suddenly in an RPG?

I heard one agent got genderbent on a really nasty Harry Potter mission,”

Harry Potter here needs to be italicized since you're referring to the series.

He then dragged Marina, who’d been staring at Dresden pointedly, out of the way and out of sight of the canon characters before (seemingly) dozing off.

Er, what? I assume he's reading the Words, given the later context clues, but then it says he's reading them by letting his eyes glaze over, so I'm not sure what's going on here.

While we’re in a badfic, if we let our eyes unfocus, we’ll be able to read the Words of the story itself.

How the agents read the Words varies; Barid, for example, hits himself over the head to see them; I've seen other examples like shutting your eyes or taking off your glasses to see them. This would be a case of PPC knowledge failure.

Why do half of these guys look like corpses, and will you stop sleeping- What is making that noise?

Hyphen needs to be an em dash for consistency.

You’re from this verse, aren’t you?

Verse should be 'verse, since it's an abbreviation of universe.

Karen here is the first Dresden Files mini

The first ever? Maybe out-of-universe, but surely in-universe people have been sent into missions here before.

The DRD will just love this, said Richard,

Missing a closing quotation there.And right after that, Marina has a line but there's no new paragraph.

I didn’t know time could be so malicious.” She added.

Should be a comma after malicious and lowercase she.

Oh, we’ll get it away from him, it could be useful.” Richard said

Should be a comma after useful.

*looks at the page number she's on* Eight?

No, I'm not going through this and spotting every single last error. Seriously, your SPaG needs work. I believe I said as much in your Permission request.

Sorry, dude, I'd love to finish reading it, but when I can't enjoy the story because of all the errors distracting me, I don't feel particularly inclined to do so.