I'll go through the edits first, mentioning only the ones that didn't work (the rest were all great):
(deleted in edit: light) ambience
Light was a better choice. Ambience means "the character and atmosphere of a place", which would be hard for a crystal to absorb.
Aermyst glanced back the way they had come.
Didn't like it; would have cut it.
in a pain-saturated groan
Ehhh...maybe. I think you can do better, though.
(deleted in edit: "If we do, ) If they find us here
Liked the deleted phrase more. It's crisper and more to the point.
Dantes stood before him.
Blocking the door.
Great edit, but I'd make it all one line: "Dantes stood before him, blocking the door." Having it in two lines caricatures the emphasis (IMHO).
"But that's a story for another time -- if your time, and your story, hasn't run out long since."
The whole last half of that sentence needs work, and the edit makes it worse, not better.
On to specific things that didn't work:
The man inclined his head. “As you will.” He brushed past Aermyst, hands dripping in white lace.
The "hands dripping in white lace" doesn't fit here. The fact that his hands are dripping with white lace has nothing to do with him brushing past Aermyst, so it doesn't make sense to cram the two together with a comma.
It bothered me that Aermyst let the spymaster see where the key was. It also bothered me that it was kept, unguarded, in a hollow brick, and that apparently any Crystalheart knows this fact. The Sacramency is displaying such trust in the fear of taboo that they may as well just leave the door unlocked. (Edit: And, having Aermyst need to "borrow" the key from a secure place would make for a tense scene.)
It bothered me that Aermyst touched a stalagmite. Have you ever toured a living cave? The first thing they tell you is
don't touch anything! Oils from your skin disrupt the deposition of minerals on the formation; in effect, touching formations kills them. Shame on you, Aermyst; you should know better.
These are, in the grand scheme of things, minor complaints. Overall, I really enjoyed this chapter. I loved the description of the hall of lights, and how it's essentially a burial ground for crystalhearts (however, this fact makes me wonder why entering is so taboo, since I would think people would want to visit the entombed souls of their ancestors). The dialog is mostly strong, the informant feels real, and while Aermyst is still doing something stupid it's a plausible and forgivable kind of stupid (IOW, the stuff that stories are made from).
I admired Aermyst for not leaving the informant. I think it's the most admirable deed we've seen from him.
The ending is tense, and I was sad to see it end. That's a page-turner ending, and I
would have read the next page, even if it were 4AM, and even if late to class. Good work!