The sound on my computer isn't working at the moment, but I'll comment on the organ part which people are talking about!
The semiquavers in the right hand certainly do look like they will be an issue. The main problem for me wouldn't be that it was such a repetitive movement, but would be the particular fourths (I'll call them fourths since most of them are, I can only see a couple of places where thirds come in at a glance) which you are oscillating between. As for the issue of the hand cramping up due to repetition, I think this could well depend on the organ; I'm guessing you would voice these on the swell rather than the great and quite a few organs have fairly 'light' swell keys which would help reduce any cramping. Most organs I have played have lighter swells, particularly in the range the right hand is within, than a piano.
As for the particular fourths you move between, there are some which look like they'd be OK; personally I find moving from a perfect fourth to a tri-tone (and vice-versa) fairly easy such as you have in bar 43. There's a piano piece by Gershwin called Novelette in Fourths so if you can listen to a recording of that (there's bound to be a few at the minute; it was selected for the new ABRSM piano syllabus I think) it might provide some ideas about what is ergonomically easier to play. I was going to suggest possibly omitting the lower note of the first of the fourths throughout the bar, but from the reviews you've had it looks like it would be a shame to remove it as it could alter the effect you create.
One more note on the organ part is the foot pedal; you keep it very low in it's range throughout the piece. There's nothing wrong with this, but the combination of stops you use could well mean that the foot pedal has a different timbre and exploiting the higher end of it's register is something you could add if you wish. It might be nice in places where the harmony begins to intensify, as the piece reaches it's climax, to add a rising counter melody with the right foot against the full force of the choir whilst the left foot sustains those long bass notes. You'd have to be careful with the intervals you chose though in order to make it sound smooth only using one foot!
It certainly looks interesting! I'm looking forward to fixing my sound and I'll comment more once I've heard the whole thing.
PS - I know it's already been said, but what a fantastically presented score! Do you mind if I ask what software and fonts you used?
I'll post more soon!
Oscar.