Wow, the soprano soloist has a range from A3 to A5 (or is it G5)? And those ornaments in the Benedictus! It would be nice if you could supply sound files, in order to get some idea of the overall sound you are aiming for. Double stops of a 10th or 11th are pretty tricky: in the Qui Sedes, why not have Vln I take the top two notes and Vln II take the bottom two notes of the four notes you have assigned as simultaneous double stops (there are others elsewhere, e.g. in the Christe eleison)? There are some iffy double & triple stops in the Quoniam. The whole note triple stop at the end of the Domini Fili (and in bar 8 of the Kyrie) might sound a little strange if sustained and not at anything approaching forte/fortissimo (is that what you intend?).

Rule of thumb (fingers? strings?) ... A double stop can only involve two adjacent strings, so (unless one note is an open string), check the stretch necessary for the two notes to be played simultaneously and realize that an interval of a 10th or 11th (or greater) requires very long fingers (fingers one and four will be required to play large interval double stops). For a tripe stop (unless the top or bottom note is on an open string), work from the string for the bottom note up on the three adjacent strings and check for fingerings.

These observations are from a very quick and rather cursory look at the score ... I've not had access to an instrument (piano/organ/harpsichord/keyboard) on which to give it a fair try. And, as mentioned above, a sound file for such a score would be a big help.

'Rule of thumb' is more of a cello thing ;-) but otherwise Chuck is right on. In m. 4 of the Gloria beat 4 you can avoid tenths and make things much easier and perhaps more graceful by having f''a'ff''a' in vln I instead of skipping to d'. At mm 11 & 12 of IV Qui tollis, it's almost guaranteed that the violins will confer and swap notes in mm 11 & 12, perhaps without even telling you so to your face.

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Clarinets in A (which have a pleasantly warm tone) are much more preferable than those in C. My own preference would be, however, to use oboes or an oboe & English horn (or even an oboe d'amore), as the double reed sound is a better match with voices.

Your bass trombone part is more than adequately rendered on a tenor trombone, if not better in the higher register; additionally, I think the lowest note in the bass trombone part is an F#, totally comfortable for a tenor trombone (especially one with a trigger mechanism, which actually allows for going all the down to the pedal B-flat tone) ... that there isn't even one low D in such a part is quite a surprise, since the work is in D. The alto trombone part does not wander very high, either, and could also be played on a tenor, but is okay as an alto part, too.

It reminds me more of the classical era, with the amount of vii/?->chord progressions you use. Mozart used them quite frequently.

However, please modify some of the jumps in the soprano line and soloist parts. No one alive wants to leap from an F#5 to a B3 to a G#5 even with rest. It will sound horrendous in even the most cantabile, trained, world-class voices.

Other than that, and some other things (primarily instrumental), I hope you show it to more people!

The offending soprano line seems like it could easily be divided between two soloists in order to preserve the original contour while easing the technical difficulties. My issues with that passage would lie less in the leaps themselves (which have plenty of precedent) but in their extreme range and jumping from the head to middle register of the soprano voice.

No one alive wants to leap

Speaking for myself, I can't listen to Domine Deus without being jealous of the Weber sisters. But people can have such queer ideas about what's singable: one composer said to me "There's a high g-flat, but I tried to make it easy for you [basso profondo] by approaching it gradually by half steps".

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While I love the bassoon and contrabassoon, I do think that adding the contrabassoon in this texture is a bit much and makes the work rather bottom-heavy.