Great find indeed.
I think there are other things like this in the Glossary, you don't write 200 pages of names and definitions without putting something juicy here and there.

Some saw the winged horror of the Horde-General flitting down and about, then darting like a sparrow into obscurity. Some saw the Aspect-Emperor slip like a coin from a slashed purse, only to vanish into the oblivion of sorcerous light. Some saw the Spear topple out into the void, trailing a rope attached to some kind of metallic coffer ...

The box is probably its source of energy but I don't understand if the box is attached to the Horn or if the Sun Lance just fell into the void.

In a realistic novel we know the range of what can or cannot happen is limited by the same physical laws as our real world.
In a fantasy novel there are much more possibilities and we need to understand how the magic and the metaphysics work in that particular world. Without that we are left with endless speculations.
e.g.: we know that sorcerers are damned and taken to Hell when they die. But who takes them to Hell? The Gods? A particular God? The Ciphrangs? Do Ciphrangs work for every God or just for the Punitive ones? And how many Hell exists? Who decides in which one the sorcerers go?
Even if I stick to the facts of Kellhus salted-Kelmomas in the Carapace-Second Apocalypse started damned ( ) if I know how these things went on.

edit: I feel like you are being a little harsh though. There are millions of papers disputing the meanings and minutae of literature taught in schools and colleges. It's part of the fun, no?

I think there's a difference though: Serious Literature scholars focus on the interpretations of clear texts.
We are more in a Gene Wolfe situation here: trying to understand "what" happened on the page.

Hi everyone, first post here but I just had to join the forum to discuss TUC   
Has anyone noticed the Decapitants entry from the Glossary?

According to Akamis, Pim told of a trip across Gedea that took the Aspect-Emperor and his travel court across the legendary Plains of Mengedda. In the deep of the night, near the end of his watch, Pim found Anasûrimbor Kellhus alone and raving on the haunted plain, alternately removing his head and replacing it with one of the Decapitants. (italics in the original)

I think it's fair to assume that for all of TSA we saw a Kellhus/Ajokli so now we need to go back and reread the whole series.
I also expected some kind of revelation about Esmenet explaining why she is the only one capable of bearing living sons to Kellhus (whose seed seems to be stronger than other Dunyain like Moenghus), either because of genetic reasons (could she be Moenghus' daughter?) or because she's possesses by some God/dess or a Kahiht.