You're crafting some truly captivating sounds here! While I'm impressed with what you've achieved, I have a few concerns regarding the practicality of certain elements for a live performance.
Firstly, let's discuss the organ melody as written. It's beautifully intricate, but in its current form, it requires an extra hand to execute precisely as you've composed it. This becomes particularly apparent when the melody coincides with sustained notes, making it challenging to perform without resorting to another registration with a separate hand (e.g., measures 7, 11, and 13). One solution could be to revoice the harmony to avoid overlapping pitches with the melody.
Secondly, I'd like to address the bass line in the chorus. While the organ part is indeed grandiose, it tends to overpower any vocal bass, especially in the lower register. To maintain balance, I'd suggest removing instances where the vocal bass descends below the staff, allowing the organ to take precedence in that range. This ensures clarity and avoids the choir struggling to be heard, particularly in passages with voiced consonants like "rie elison."
Considering the organ's formidable presence, it's crucial to carefully select registrations to prevent it from overshadowing the choir, especially given the nature of the text. The MIDI rendition might suggest a certain balance, but in reality, a choir of fewer than 60 singers would likely necessitate adjustments to the organ sound to complement the vocal parts effectively.
Regarding dynamics and registration shifts, it's essential to clarify your intentions to the organist. While you've indicated four dynamics, it appears you're aiming for a consistent principal classic organ sound throughout, as hinted by the use of crescendos at each dynamic shift. However, it might be beneficial to specify desired registration changes either through text near double barlines or as a footnote. For instance, transitioning to a lighter, more flute-like registration when the choir enters could enhance the overall blend and balance, particularly during quieter passages indicated by decrescendos. You could then increase the intensity between verses.
Overall, your composition shows great potential, and a few adjustments to accommodate practical performance considerations could elevate its impact significantly.