I'd just like to point out that trumpet in C is not absolutely rigidly required. Stravinsky wrote the ballerina dance in Petrushka in Bb (not sure about he rest of it cause I haven't seen it yet). So was the 1st movement of Tan Dun's piece for the Youtube Symphony Orchestra (don't know if you've heard of it), and the Arrutunian Trumpet Concerto. Carmen was written for A trumpet and Tchaikovsky (and many other romantic compsers) wrote for A trumpet several times too. And all these examples are from within the last 200 years.
That said I think C would be better because the American standard is C trumpet (European orchestras may use a combination of Bb, C, and occasionally low F trumpets within the section but that is up to the players themselves to decide since professional trumpeters are expected to be more than decent at sight transposition)
Now about the music, you notated mourning in the beginning but, to me, it didn't sound very mournful. I can't really explain what it did sound like, but I feel that the dotted rhythms and the placement of the sixteenth notes make the piece sound a little too upbeat but yet weird. I think on a more basic level your dissonant chords are not jiving. IMO the way you wrote it makes the music sound loose and unfocused.
And that's about all my limited experience allows me to communicate in words. Good luck on your first symphony though. I've still yet to successfully write for larger than a brass quintet.