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"...
My (indult) parish has used Mozart Masses for Christmas and Easter the past few years. They don't quite overwhelm, but liturgically we lay it on rather thick anyway. It's a close call. I wouldn't mind trying Palestrina for a change; maybe I'll find a way to mention this to our pastor (a music major from Northwestern).