Hey Cream, I'm glad you are developing an interest in the 'King of Instruments'!

May I offer some suggestions?
I think at this point, you should leave the registration to your organist friend. If he is advanced enough to play your composition, letting him use his own approach would yield the best results. There is nothing wrong with a basic outline of course, but any organist worth his salt can make do without a stop-list.
Something else to consider is the actual playing of the organ. When I compose for the organ, I try to strike a balance between complexity and reward. Your piece is playable but would require a good amount of work to be played properly, and I don't think most organists would bother.
I'm not trying to be harsh, (I'm working on the same issue

) but perhaps a reevaluation of the piece could yield better results? Measures 49-50 are very interesting to me. Could you expand and re-write the movement with more of this kind of structure? A simpler tempo would do this piece, and the performer, a great service!
Would your friends be willing to sit down at an organ and go over the piece with you? Perhaps let you play it? That would be a wonderful way to learn more about organs and organ composition. Give it a try!
Keep working on it,
Justin