Cat - I think the majority of cantors do not rehearse a day or more before the mass. They usually only have enough time right before. That is a problem with volunteers. They are giving the time that they can afford, so we can’t expect them to give more than that. My old parish had cantor rehearsals on Saturdays (and, yes, that is unusual - never had it before in any other parish), but I never could go because of my own schedule with other gigs. But again, the organist didn’t have to worry about me since I was a musician. We hardly ever rehearsed with each other, just talked about what we were going to do about 15 minutes before the mass either in the sacristy or in the loft. He was more worried about the other cantors who didn’t read music and needed help.
I mentioned this in my other post, but I think this may help your situation with the cantors. This is something I know takes place in a number of parishes. Since you are usually there an hour before, you have plenty of time to practice your repertoire before anyone arrives - including the cantor.
For the cantors, they should already be warmed up before they get there. And usually cantors should have the music ahead of time to look at prior to the mass. I know in the past if we had to run something really quickly before mass, but it was close to the start of mass, I would sing really quietly so that only the organist and myself could hear in the loft. This would usually be for the psalm so that the organist would know where I phrased the words for the chanted verses.
If can also offer some additional advice in regards to being a guest musician… I am often a guest musician because I am hired to cantor for funerals, weddings, as well as just special masses at various parishes. I also know what it’s like to not be able to get in anywhere since they are not my parishes. For that reason, I try to always have my music prepared ahead of time. I warm up at home, then in the car while I’m driving there. If the organist agrees to come an hour ahead of time, we will meet and rehearse while the church is empty.
If you know that you can only make it about a half hour to 15 minutes prior to the mass, just be prepared ahead of time so that you don’t really have to do too much (if any) practice at the church. You should have the music way ahead of time to practice at home. If you can get to the church only 15 minutes before mass, just lightly touch the keys on the piano so that no sound is being made - just so you give your fingers a little work out and remind them where you want them to go.

But always prepare yourself as if you will either not have a practice room or will not have time to practice at the church ahead of time. I’ve always found that helpful for myself.
I say this with love and to hopefully help because it does seem like you are frustrated with your own situation. From how you describe your city and situation, it seems like a real downer. But, as my mother always says to me in her constant, optimistic, can-do attitude, “Where there’s a will, there is a way.” Quiet before mass (even 10-15 minutes prior to mass) can be achieved with very few adjustments in how one either runs through the music or prepares ahead of time before even getting to the church.