I’m impressed! This is really well written. I have a few comments; take them with many grains of salt as most are stylistic and I’m not well-versed in the style of this piece—so they’re mostly blind and possibly irrelevant.
Firstly, make sure you have a little note somewhere that says that the score is in concert pitch. I'm certainly not complaining though; it was far easier to follow the score not having to read a whole bunch of transpositions at once.
Long ride for the solo part here. Be aware that some of the double/triple/quadruple stops and leaps you've inserted will probably be very difficult to execute at performance tempo. I'm sure your dedicatee is very talented, but make sure they're able to pull these off while maintaining the integrity of your markings (and of course good intonation and sound quality). I'm not really worried about impossibility, though, since I know you play the viola. One spot that caught my eye at the get-go: m. 19-23. Those leaps are really up there, and probably nigh impossible to execute while playing pianissimo. And then in 22-23, even though that passage is easier than 19-20, it's still going to be really hard to play quietly. Same for 64-66. That crescendo will probably end up quite a lot louder than mezzo-piano. The triple stops at 53-54 and 57 will also be very difficult just in terms of fingering and intonation. It also looks like the first two triple stops in m.70 are impossible. The first one will either require Db on the G string, A on the D string, open A, and B on the E string, or else a leap from the D string to E mid-double stop. The latter of these probably isn't feasible. I'd add a second A to the score to make it clear (if that's what you're going for, that is). The second triple stop requires both an Ab and Db simultaneously. Since these are on the G string, that isn't possible unless you notate the Ab as a grace note to indicate that it isn't part of the chord and should be played before the Db and Bb. But be aware that the triple (double?) stop won't sound as smoothly this way. But that's better than being impossible, right? As for m. 88, the G and C# double stop is impossible, no way around it. I might stop focusing (or focus less) on the solo violin with my comments for the rest of the piece, so I'd advise you to do a really good proofreading session for the violin part and check for feasibility. I probably missed other impossible double stops since I wasn't particularly looking for them, so make sure you screen the whole thing. On the high notes, it might be a good idea to put in harmonics (natural, not artificial) where you can. I know you've done this in several places to achieve a certain color. But putting them in more places (especially faster notes) will improve the intonation, won't change the tone color (too much, at least), and will make some of the high sections much easier. I'm writing that looking at m.109, where all of the notes could be harmonics and the part would become much easier and probably sound quite a lot better. Another thing; most of the difficulty in the piece is the double stops and the leaps, and a great difficulty they are. If I were your dedicatee, I would appreciate varied items of difficulty (for example, a fast passage could work well in the character of this piece). Also (this I'll talk about more later), expressiveness is my favorite challenge as a violist. Technical difficulty is all well and good, but there's not as much interpretation in that. Make sure you have plenty of chances for the soloist to express themselves as a musician--really, that's the most important part of any concerto--and not just show off their quadruple stop and shifting skillz. I enjoy the chords, though. Don’t mistake that for me saying I don’t.
As for the style, I think you've done really well. I can't say that I know a lot (anything, really) about this style and won't pretend to, but here's one thing I do know: going back to one of my comments on the solo violin earlier, there need to be so many chances for the soloist to be expressive. That looks different for everyone. After reading your response, it’s apparent you’ve put a lot of thought into constructing the thematic material of the concerto. You’re fortunate to have a dedicatee for this concerto, which means (and maybe you’ve talked about this while meeting with them?) that you have the opportunity to insert passages exclusively suited to their creative voice. You said that the violinist isn't a huge fan of your compositional style. Adding more “traditionally” melodic segments to the soloist part could be a great way to make him/her enjoy the part much more and balance the style you’ve written in with their playing style. Additionally, that could even work really well as a way of contrasting the ensemble and soloist and providing opportunities for musical dialogue. Just food for thought.
You also might consider adding just a few places where the harmony is more traditional (I’m saying that as someone inexperienced with this style, so again, grains of salt). I mentioned to you that post-romantic music is so enjoyable for me because the modern compositional techniques coupled with traditional elements make for more expressive, more impactful (to me) music. I'm not telling you to X out all of the long dissonant passages, but I would have enjoyed places of harmonic resolution (in terms of modern to traditional, not necessarily a cadence) Even if you just have these a little bit, everyone will be happier: the soloist, the orchestra, the audience (that's an important one), and even you, I'd wager, because the musical style will possess greater balance, making everything in the piece more meaningful. Think of whyThe Rite of Spring might be regarded as one of the greatest musical masterpieces of the 20th century even by general audiences—it’s very dissonant, very polytonal, right? For me, Stravinsky's masterstroke was to provide relief from all of the hard-on-the-ear material. He did this within movements, but also provided long sections of surprisingly consonant harmony. My favorite movement of the Rite is 'The Round-Dances of Spring’ because of the relief it brings—that makes both the traditional and contemporary aspects of the piece more impactful. I’m sure that his contemporary writing was also brilliant and there’s something to that too, but I’m no expert on that.
Quinn said that the ending is too bombastic for the translucent texture, and he's right. I actually really like the end though. If you’d like to keep the end as it exists in this score, I just have some brainstorm-y ideas. I would vary the texture quite a lot more. Give more textural time to the woodwinds and brass. Based on just the sound of the MIDI recording, the concerto is very string-heavy (I’m not saying that after intensive score review); that isn't a bad thing (and a live performance would probably yield a very different texture), BUT even MIDI playback can make distinctions between different textures! I don’t even now how applicable that is to your piece, but it’s always good advice so I’ll leave it.
Also, I missed out on a climax, which would make a louder ending smoother.
Another thing: where are the color and character changes? Perhaps it's just because I'm not used to this style (or maybe just MIDI), but I couldn't really hear any.
Anyway, I think that constitutes most of my feedback. This is really good, and I'm excited to see how it turns out!
Jacob