Thank you so much for your detailed and genuinely interested comment! I'll try to respond to a few points in order. In the "main theme," the tremolo-style arpeggio is played on two kennel (an Estonian instrument similar to a harp). And "aleatoric" is indeed the right term to describe these particular string movements—look it up on YouTube; it's a unique technique, especially for building tension, in my opinion (I love it in certain parts of *The Lord of the Rings*... if I'm not mistaken, it's used when the fellowship is standing before the entrance to the Mines of Moria. That whole scene features aleatoric music).
For the choir library, I used EastWest's "Diamonds." I highly recommend it, especially because you can write custom words or phrases. In general, EastWest libraries are very good for choirs; there's also a newer one by Native Instruments that looks intriguing, though I haven't tried it yet.
Lastly, I really appreciate the compliments on the tracks and the soundtrack as a whole, and I'm glad to have been able to inspire you! Thanks again.