Is this Offertory the «Super flumina Babylonis»? I guess that if your choir cannot learn the gregorian chant, then Rice's setting for two voices (TB) and organ is also out of question, since it is also too difficult...?
Propers could be sung by a choir in Anglican chant also.
Single cantors often sang the proper (and sometimes the ordinary) in the old days. If your choir CANNOT sing the offertory, I think you would be justified in having the cantor sing it alone. The psalm tone offertory would serve if it were followed by a motet; otherwise, it will not cover the offertory action.
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