We don't know the specifications of your organ.
We know nothing about the room (acoustics, etc.)
We don't know how your people and your schola sing and how they respond to different registrations.

Don't be afraid of a plenum combination.
Don't be afraid of soft combinations.
For the pieces you have listed above, choose a different sound (one that sounds right to you) for each stanza.
For the descant stanza of Adeste fideles you will need to play the melody on a trumpet stop in one manual (left hand in the tenor voice) with an accompanying sound on the other.
For the last stanza of Adeste fideles use 'full organ'.
For Mendelssohn use a very full sound for the first and final stanzas and perhaps a less than full sound for the middle stanzas.

In choosing registrations, be careful always to be able to hear your choir and the congregation - but this doesn't mean that you should be afraid of a bold registration.

the instrument is 2 manuals
Allen Digital Computer Organ

We require a complete specification ie
each division identification followed by all its stops/tabs/couplers

Oh yay, an Allen. /purple

I would echo what MJO said, and I’d like to add some Allen-specific advice, based on many years of dealing with toasters.

Due to the screaming nature of Allens, because of their puny 8s and deafening 4s and upperwork, try to follow this scheme in your buildups, from softest to loudest (you might set this sequence on the generals for handy use).
1: single flute or string on each manual, pedal bourdon, sw/pedal
2: The next step: MANUALS COUPLED and play on great. Right away. You need to atone for the narrow and shallow foundation tone every way you can, all the time.
3-4: Add more 8’ flues until you have all 8s on. Make the sound broader, not more strident, at this stage, and open up the swell box(es, hopefully) for more. Sw/ped, Gt/ped
5: Add 4’ flutes and more 16’ in pedal to balance
6: Add swell oboe
7: Add Sw Principal and trumpet, shut swell box, keep great box open, if possible
8: Add Sw 16 reed and mixture
9: Add Great 16’ flue and Great 4’ principal
10: pedal 16’ reed, maybe great 2 and mix; shut great box down a bit.

The idea always is to use the foundations of the great to atone for the wimpy foundations of the swell and general harshness of the organ, and then add color via swell reeds and 4’ and higher stops, the volume of which can be tempered by shutting swell. This is in fact a poor simulacrum of the way the Brits operate, with the great 8s used for support and sturdiness and the powerful swell under easy control via divisionals and swell pedal.

If you’re unfortunate enough to have only one swell pedal to control the volume of the whole organ, I apologize. The above will still apply, but the outcome will be less happy.

Remember that if the sound is grating or displeasing to you, it will be the same to the congregation. Trust your ears and don’t create scandal for the faithful. :)

Throw the celestes on for the larger registrations; maybe around number 8 of my scheme. This will ever so slightly warm the sound and begin to approximate the spread of a real organ, with its never-quite-in-perfect-tune-ness. But the noise of the reeds and other loud stops will hide the celestes decently, such that the choir won’t have tuning issues.

Not sure how new the Allen is, but almost all of them have some way to get a frighteningly loud Hosanna Horn for the melody of the penultimate verse of Willcocks Adeste Fideles (where DW writes “Tuba”). This may be via the alterable-voice punch cards in a very old model, or on a MIDI module in a junk drawer that pulls put beneath the console. If you google the model number of the organ (or the year if not known) plus “alterable voice”, you’ll find guides to using this system. Or there may be secondary stops (engraved in red on the tablets) that activate with the “second voices” switch. One of them will be a big trumpet that will suit the purpose well. Just watch how you set up your couplers so that you don’t accidentally couple the solo to the manual where you’re accompanying.

Good luck!

We require a complete specification ie
each division identification followed by all its stops/tabs/couplers

I do not have a specification available nor regular access to this instrument. If I did, I'd be able to offer that. Please forgive this inconvenience.

Thanks, overall, for your general suggestions. Once I get serious quality time with this organ I can map out my actions.

Gamba,

Your detailed response concerning the ways to achieve certain sounds and avoid others has done a great service to the readership hereabouts. Thank you.

(Purple) Avoid any registration on an Allen. Turn off and proceed to the next job. (/Purple)

OK... all kidding aside, just watch the volume so it doesn't overwhelm the choir or the congregation. Stick with the principals for congregation and flutes for the choir.

2 Corinthians I:

And whether we be afflicted, it is for your consolation and salvation, which is effectual in the enduring of the same sufferings which we also suffer: or whether we be comforted, it is for your consolation and salvation. And our hope of you is stedfast, knowing, that as ye are partakers of the sufferings, so shall ye be also of the consolation.

Just trying to help people in their misery.