Last week I spent an hour on this piece with the chorus, going over the finer points of pianissimo singing and the pronunciation of Germanized Latin. If I did my job correctly, it must have been an excruciating experience for every one of them. One later admitted to murderous thoughts, and I knew that I did right by my job.
It's so easy to sing Mozart, notes that fit the voice so naturally as though he tailor made it for singers like a glove. And yet there is so much more to be mindful of. As has so often been said (because it's true), Mozart is both the easiest and most difficult composer to play. Any musical five year old can sing the melody from Ave Verum Corpus, but it takes another fifty years to get all the subtleties of the score exactly right. The great Artur Schnabel would say that it's a piece that's too easy for children and too difficult for artists.
It's one of those rare 'perfect pieces.' Three minutes of music in which every note is completely necessary. But it's also one of those pieces that's better than it can ever be performed (Schnabel again). If you like, you can sit there for as long as the chorus can maintain its ability to sing pianississimo and weigh every singer within every chord for exactly the right color and blend. But you'll never get there.
Instead, all you can do is make the singers aware of just how much delicacy goes into the piece from beginning to end. Work every phrasing until they understand that this is the kind of piece that only works if they're listening as closely as you are. We'll probably keep this piece in the rep for quite a while, who knows how long ultimately? But in the meantime, I hope I still have a chorus that'll forgive me for putting them through the ringer on this.