last two weeks of choir

so, like most church choirs, mine has been slowly dwindling since Easter.
Throughout the year, I've lightly threatened them that if they miss rehearsal, I will be calling them to make sure that they're doing alright and not in the hospital or something, cuz, really, that's the only good reason to miss rehearsal, right? and those present always agree!
and of course whenever there is a particularly light week, the people who were there later want to know if I carried through on my threat, but I never have.
until yesterday! there were only 4 ladies at rehearsal, and really, they are those who don't need to rehearse much! so, yesterday, I called about 5 people. I left some messages, expressed polite concern that they had missed, hoped they were doing alright, and mentioned that the choir would only be singing for 3 more weekends, so I really hoped they would be able to make it to the last two rehearsals. A couple people were surprised that we weren't singing through the summer, and most sounded like they certainly would make a priority of coming to the last rehearsals.
So now here's my quandry. I had planned on learning a rather difficult but gorgeous 4-part Ave Verum for Corpus Christi if I had had a couple guys to sing bass at least (I can sing tenor when necessary, and my sopranos and altos are pretty competent!) but I decided against it with only the 4 ladies. We worked on our 4-part piece for next Sunday, and even if just these 4 sing the soprano and alto parts and I play the organ, it will sound alright.
so anyhow, now everyone is going to come to rehearsal for the next two weeks. But I think that 2 weeks might be too short for most of the men to learn their part on this difficult piece, so it will be kind of like, "why are we here? we're not doing anything hard...?" I don't think most of the choir knows the chant Ave Verum, so I'm thinking of just teaching and using that. At least they'll know the words for next year, right?
my point is, I'm afraid of not challenging them enough(which I haven't been able to do recently cuz they haven't been coming to rehearsal regularly!), so they are reluctant to come back (like in the fall?)
we'll see. I'm learning. it's still my first year as an actual Music Director...