Of course you may do what works for you, but a couple of things I use that may be helpful.

Those low notes: If you can't play them on pedal, and I realize sometimes that's nearly impossible, pull on a 16' on one manual and play the low notes with your left hand. If you don't have a manual 16' but only a coupler, you can play some of them and pick up only the really low notes on pedal that run below the range of the coupler.

I have to say I have seen plenty of those piano scores. If you try to play all those notes on organ, it will be thick as mud. You will have to leave some notes out or split them between hands and feet. Leave them out, it's easier, will probably sound better. Generally, if you get the SATB notes on accompaniments, you have covered what the singers need to hear. They won't notice the missing inner voices.

Shoot the choir director. Save everyone grief. Just kidding, but sometimes they don't realize the difficulty.

You don't have to lose your job over this, just manipulate the organ to your advantage.

OK, that was more than a couple of things.

Part of my Service Playing training was to be able to render pianistic accompaniments as idiomatically as possible on the organ. I believe the two I was assigned were the Beethoven Hallelujah (Mount of Olives) and the Hallelujah Chorus from Messiah (the Schirmer edition, specifically—I use the more recent Novello edition now that envisions the organ being used). It was a pain, and I didn't enjoy it, but I learned a lot in the process. Of course, the more practice you get will help hone the skill.

If you're not working with an organ teacher currently, have you considered seeking one out for the purpose of working through these pieces? If you are, maybe you'd bring these pieces to your lessons to work through.

As an aside, while the accompaniment for the Alstott piece you mention says "Keyboard", it is idiomatically written for the organ, and OCP uses the organ in it's official recording. Maybe you've encountered a different arrangement? This is the one I'm familiar with:

http://cdn.ocp.org/shared/pdf/preview/22701z4.pdf

https://www.youtube.com/watch?v=u9wHr_DPUhY

Marc

YCG

Apologies that tragedy has befallen you so early in life.

The best thing you can do with the piano accompaniments is to ignore them completely. They depend on the percussive nature of the piano to hold the groove, and the damper pedal to let the complete chord ring as you do the left-hand idiot wobble (eighth-eighth-quarter). Without those things, they’ll sound empty and utterly stupid on the organ.

Just get a pen and copy the chords above the melody line in the hymnal/missalette/whatever. Put the melody in the right hand, either on a solo registration on a separate manual, or in the soprano voice if playing on one manual. Realize the chords in the left hand, then use the pedal to supply the rhythm: play there like the typical awful church bassist would: on the first and last beat of every measure, with as much 16’ foundation tone as you have.

Something like this (but with less trem)
https://youtu.be/SX-3KFJQI5I

This way, you’ll have a clear melody, sufficient harmonic support to make it clear what the tonality is, and an easily-felt rhythmic pulse in the bass to keep everything together.

I don’t know what your theory background is, but I hope you have/will study figured bass, counterpoint, and harmony – these disciplines will serve you well such that you will one day be able to harmonize any melody at sight, in a convincing way. Life is so much better when you don’t have to carry around twenty tons of accompaniment books just to accompany one garbage G&P tune.

And then study jazz, and then you can have the fun of counting how many tritone substitutions you can fit between the eagle’s wings.

I did forget to mention those chords. Thanks Gamba. They have been a lifesaver. I never learned that wretched accompaniment to Eagle's Wings when I have played for a funeral - only time I will agree to play it. I play chords in left hand, and melody with the right. It's a good skill to learn.

Amen! I have been tempted to put on the tremolo and play those arpeggios. I didn't do it because I was afraid they would like it.