It sounds like you've been listening to Schnittke. (: I like what you are doing musically very much.
A few notes on vocal music notation protocol:
If you combine voices onto fewer staves (SATB with up-stems and down-stems instead of breaking it out into S1, S2, etc.), it will be easier for the singers to sight-read: fewer page turns mean they can read ahead more smoothly, and vertical condensation means they can more easily check themselves by comparing with other voices.
I would also suggest including a piano reduction as the bottom staff for use in rehearsal. It's easier than reading an open score and is pretty much expected for a cappella works these days. (Mark the piano reduction "for rehearsal only" and mark the whole piece "a cappella" just under the title.)
The phrase markings that you've included across multiple syllables are not really done in vocal music. Ex. T1 at measure 15. In vocal music, slurs imply multiple notes to a single syllable. They are used to help singers keep their eyes on the notes instead of having to constantly look down at the text to see if the next syllable has arrived yet. Just write an expressive direction instead: legato, or as smoothly as possible, or whatever you're looking for.
I'd change out the directive about Bel Canto at the beginning for something phrased in the positive instead of the negative. "Without vibrato, simplice, ethereal, floating, etc." sounds less preachy. I'm performing a season of Baroque work at the moment. It's just understood that you minimize vibrato and sing with a lighter tone for Baroque period. If you've been exposed to lots of Bel Canto, it may feel like that's all anyone does, but there are certain classical vocal genres where it is completely stylistically inappropriate, so no one does it. Don't worry, this won't be a novel concept to experienced singers, and inexperienced singers aren't going to be performing your piece.
You will definitely turn some heads with your treatment of the text, so I'd think hard about your decisions. (It's a contemporary work, so you can do what you want, but if you weren't meaning to be pushing boundaries, you should know that you are!) It's normal for the text to be broken into phrases so that different voice parts sing different parts of the phrase at different times. It's normal for one voice to start a phrase and a different voice enter and finish the phrase. It's not normal for a voice to sing only some syllables out of word. Ex. T2 and basses at measure 16 "du... ve... est." You can do it, but know it's going to raise a few eyebrows. If you wanted the same effect with a less revolutionary feel, you could make that particular section "duc (half note) - tor (quarter note on the same pitch). The consonants would line up with the T1s, so you wouldn't really notice them, and you could use dynamic markings to minimize the T2 and basses even more.
Different publishers have different guidelines for how syllables are split, but going with the way words are split in a standard dictionary is generally a good start. For your text you'd probably come out with et Do-mi-nus qui duc-tor ves-ter est ip-se e-rit te-cum non di-mit-tet nec de-re-lin-quet te no-li ti-me-re nec pa-ve-as. Use commas any time the same part repeats a word or phrase. (et, et, et Dominus...)
That's everything I can think of. It should be lovely!