"some of the hymns were in different keys, and sometimes different time signatures on the opposing pages."

Somebody was assembling the mass booklet and found the version with English words in one key and the version with Spanish words in another key. Xeroxed 'em, slapped 'em in.   Assumed it would not matter to the congregation, who follow the words and perhaps the note lengths, but pay no attention to key.

Time signature is much the same - the congregation sings it the way the choir sings it.

I assure you the choir and musicians all played in one key.

I have a friend who plays flute in church, and he says they can play Hymn A to Tune B, and nobody ever notices that the notes are all wrong. (This would not work in my church. We're Lutheran.)

Joe, I know what you mean. When are music publishers going to figure out that most Americans cannot sing high D and E?

Tell your screaming pianist about Noteworthy Composer. That is, if she can fake things.