First off, your main theme is very well written, it's recognizable from the first few notes and conveys (to me) a distinct sense of sorrow, however you could've varied it more. Throughout the piece the theme is only really played in a detaché manner or by the piano, I suggest you try exaggerating your articulations much more to make the most out of it, for instance, try having the viola play the theme all legato during the second iteration, as for piano, make use of its extreme range, I know octave shifting sounds cheap, but just moving the theme to the 2nd bottom/top octave can make it sound much brighter or darker.
I feel like this was held back somewhat by structure, this has to do more with my taste, but I think it'd help if you built your piece around your main idea with 3 or 4 iterations of your main melody separated by solos or duets, I suggest you listen to baroque concertoes for idea of how this works (here's one https://www.youtube.com/watch?v=QPba-i26YNA, also try the four seasons).
I also read along with your score (thanks for providing it!) and since you asked, yes the viola part is all fine, a soloist would have no problem with the vast majority of this piece; the double stops are all carefully written and the viola's full range is used with proper consideration.
Well done! keep up the good work!