Thanks for your clarifying and extending remarks. This has turned out to be a really good thread.
As you know of course, tradition holds that the very first accompaniment of "Silent Night" was by guitar, for lack of a functioning organ.
I'll annoy you by saying that I don't think Schubert's "Ave Maria" belongs in church at all. It's a very nice composition, but really, it's a religious song, not a proper hymn. And again, the problem of technical difficulty presents itself. It's not really choral music, and is almost always performed by a soloist who needs some degree of virtuosity to bring it off -- and solos being individualistic, they undermine the sacramental language of liturgy's call to communion. The whole "Charlotte Church" approach to liturgical music strikes me as radically un-Catholic.
And Bach? Oh, dear. Though a Lutheran, he was a devout, believing Christian, which puts him ahead of Mozart right there. His sacred oratorios are not liturgical, so there's no risk of their turning up during Mass (they have some fine hymns embedded in them, which can probably be made acceptable for Catholic liturgical use). The B-minor Mass is one of the most glorious compositions there ever were or will be, but IMO its baroque splendor overwhelms the liturgy: sung in the course of a real Mass, I fear it would be all about the performance, rather than what's happening at the altar. And the instrumental music? My parish has a goofball organist who every year rolls out the Toccata & Fugue in D on the Sunday before All Saints'. Why? Is he suggesting a halloweenish, Phantom of the Opera association? Why? Once again, it's a piece I love, but has no direct bearing on what what happens at church.
Yes I know about Silent Night's genesis--not really the reason I used the example, but...
Schubert also wrote a couple of very nice Masses (one in G major is eminently singable by a parish choir.) The question of solo work, (like the question you raised regarding the B minor Mass) is one of context, I think--that is, bearing in mind that the REAL purpose of music in worship is 'to glorify God and to raise the minds and hearts of the Faithful to God' (Pius X)--then, if the soloist moderates the voice, one can argue that the music's REAL purpose has been fulfilled. However, I do not like a Mass-full-of-solos; again, 'context.' Thanks for mentioning the category of 'devotional' music, into which the Schubert Ave fits. Pius X was (IIRC) of mixed mind on 'devotional' stuff for use during the Mass. I think he wanted to rely on the educated and informed judgment of parish musicians--but then, he also relied on Bishops to SEE TO the proper education/qualification of church musicians--and you know what happened.
As to the B minor (or Beethoven's Missa Solemnis)--you are correct; it's Mass, not a concert with obiter dicta from the priest. Thus, even the Mozart Masses are likely far "too big" for a typical Parish Sunday Mass. HOWEVER, they are not "too big" for a Pontifical High Mass with smells, bells, processions, etc., etc. Context counts.
Bach's orgelmusik is generally highly-developed intellectual exercise, written to stimulate the brain (sometimes it works.) IMHO, using it pre- or post-Mass is fine, although I have reservations about the Dminor fugue precisely because of its secular over-use. OTOH, the D Major fugue makes an excellent recessional, and using JSB's variations on "Sacred Head" during Lent pre- or post-Mass is worthy of approval.
But yes, JSB's music is not "musica sacra"...