Des will speak for herself, but allow me a couple of 'background' observations.
Methinks that you have read far too much of the "modern liturgical" literature which sets up a false dichotomy: that one can either 'participate' or 'listen.' To one degree or the other, this dichotomy has been used to browbeat otherwise well-meaning people into "sing, jump, say, repeat, shake, rattle, and roll" activities---none of which are necessary.
The phrase "actuosa participatio" used in the VII document on the liturgy does NOT mean "active participation." Rather, it means a metanoia--conforming one's mind to the action of Christ in the Sacrifice of the Mass, and His prayer life.
This 'conformance' allows for several possible routes, among them being intense prayer (with the priest simply by following the text he reads,); one's own self-offering at the Offertory; and thanksgiving after Communion.
Now the nature of genuine liturgical music is "that which elevates the minds and hearts of the Faithful to God" (Pius X.) In brief, this music MUST: 1) use acknowledged 'sacred' texts (eg from the Scripture or text of the Mass;) 2) be 'holy, beautiful, and universal' (also Pius X) and 3) thus be truly art.
Properly speaking, then, this music SHOULD transport the listener (who listens actively--thus participates actively) to God, or at least FROM the world.
Therefore, liturgical music as defined above, serves the purpose of encouraging or helping the listener "to raise his mind and heart to God."
In fact, this IS ACTIVE PARTICIPATION, or conforming one's mind to the Mind of Christ, as Beauty is an attribute of God.
Please pardon me if this is a bit abstruse, but it is important.
It is also important to know that the church choir serves BOTH as representatives of the laity AND as representatives of the angelic choir (cf. Ratzinger and Pius X, Pius XII.) Thus the choir (and delegated singers therefrom) are singing not only 'for' you, in which activity you can join mentally, but 'to' you (eg at the Sanctus, which is a DIRECT quote from the angels' words as recorded in Scripture.)
All to help YOU "lift your mind and heart to God."
SO much for false dichotomies.
I utterly and totally loathe and despise clapping in church. Fortunately our congregation never applauds sacred music during the Mass. Unfortunately the line has not been held for awards, appointments of vestry, etc. that occur either right before the service begins or during the sort of lull right after the Peace that is used for announcements. (My personal opinion is that announcements should be made before the service or afterwards. Too disruptive during, even after the Peace.)
Now for your other question. Two aspects: practical and philosophical.
1. Practical. The church I now attend was founded in the 60s, with all of the touchy-feely nonsense that pertains to that era. One of the original rector's less than bright ideas was based on just your objection - that a "trained choir of unsanctified singers" detracted from community worship, and that all singing should be congregational. In practice, it meant that nobody sang. Part of the problem was that the acoustics of the very radical 60s-architect-designed building were so rotten that you could only hear the person to your immediate left and right. If THEY weren't singing, you got embarassed and mumbled to a halt. Since nobody could hear anybody else, EVERYBODY mumbled to a halt. Thankfully when our current choirmaster came on board he got a first class choir organized, and then agitated for an acoustic reworking of the sanctuary that turned into a general renovation. The congregation is much happier now (they can hear each other, and they have a choir to lead the singing.)
2. Philosophical. The problem with music is that to be well done (and a work done in honor of God SHOULD be well done to the best of ones ability) it requires talent, training, and intense concentration. Music that is simple enough for the average non-musically-trained member of the congregation to sing along, will be less than the best that the trained singer can offer to the glory of God. The best composers of sacred music have offered to God music that can ONLY be sung by a well-rehearsed group of trained singers. To discard the choir would be to discard not only an entire body of music by Bach, Mozart, Palestrina, Byrd, Dowland &c. &c. &c., but to discard the God-given talents of those who sing.
That said, the singers have a responsibility to sing the music sensibly, well, and sensitively to the role they play in the service. Think (for an analogy) of the difference between a singer at a baseball game who gets up and sings the "Star Spangled Banner" clearly, plainly, on pitch, and as written, and the hatchet job pop singers who get up there and howl, hang on the high notes, add all sorts of trills, octaves, and other flashy fireworks not in the music, and generally mess around in order to show off their "voice" or their "style". The first is honorable and does honor to the National Anthem, the second is not because it moves the focus from honoring America to paying attention to the singer. Same rule applies in church.