You know I love this story thematically, but I'm also very impressed with the execution. You undertook a really difficult thing with the pronouns, and it works. Even your use of gender-neutral pronouns, which I usually have a lot of trouble with (not in theory; I just find them clunky), is great -- whereas I've used "it" in the past to flatten unembodied Castiel out into an incomprehensible alien, your "ze" beautifully maintains his personhood and multifariousness.

Also:

"You have to know," Dean said, voice hoarse. "You have to know that I—"

Aw, Dean. Actually, this reminds me that I also love how you've handled Castiel's memory -- I'm always hungry for fic about his four-dimensional perception, and even if that's not the focus here I enjoy that tidbit.