Congregation Participation (Grr)

At choir practice last night, we were practicing Mozart Missa Brevis in D. When we came to the Agnus Dei, we went through it until we finished the first "miserere nobis"--and stopped.
Me: Why are we not singing the rest?
Choir Director: The bishop (we sing at a cathedral-KA) thinks that the rest is too difficult for the congregation to participate in and that it is too long.
Me: (grinding teeth) Since when is Mozart too long? Isn't that a contradiction in terms? And how on earth is the congregation supposed to "sing along" with a complicated four-part Mass that most of them have never heard?
Choir Director: (Weak smile)

Now, I have several questions. First, why in heck would we be singing Mozart's Missa Brevis at Mass in the first place? It is clearly not liturgical (look at the nearly operatic soprano parts, among others). Secondly, IF we are going to sing it at Mass, why would we have the congregation sing it as well? This is not Mass of Creation, folks, this is MOZART. You can't just "sing along" to MOZART without an unmitigated musical disaster. Thirdly, IF we are going to sing it at Mass AND we are going to have the congregation "participate" (Heaven help us all), why would we further murder it by cutting off two thirds of it? (As a side note, I suppose a snide answer to the last question would be that at least we don't have to listen to the congregation murder the remaining two thirds.)

And WHY am I considered unreasonable and nitpicking for asking the questions stated in the above dialogue?

Probably because beauty is in the ear of the beholder or some such nonsense. I might go for that (okay, not everyone likes country music and Palestrina) in some cases, but no one but a ranting, raving, ax-wielding lunatic could possibly think for a second that sing-along Mozart sounds "good." Or, for that matter, that one of the most beautiful Masses EVER (however unliturgical) has to be cut short because it is "too long."