Sounds like a tough dynamic. I was a singer in a couple of bands and played in a few others and I didn't encounter that situation, but I will say that I have been the singer in a position where I felt really uncomfortable and it was a pretty miserable experience. I mean, if it's a disaster you're kind of the defacto "band leader" by perception whether it's true or not. They don't call singers "front men" for nothing. We were all on board, but in my younger days we played a party or two that we weren't ready for and it kind of sucked. I mean, the crowd was nice and all (parties are a great atmosphere to "get your footing" because people aren't there for a show they're there for the party and the attitude is generally much, much more forgiving), but it just wasn't any fun and it was way more stressful than it needed to be.
It sounds like you're outvoted on this and they're not really going to give to your concerns so you have a choice. I don't know if these are your good friends or if this is just a thing of people coming together to make music who don't really hang out otherwise, but on one hand you can bow out. If they are your friends, hopefully they can understand that you just aren't on the same page and respect it. Even if you don't really know them, same thing. On the other hand you can stay and do the best you can and try to get some good experience out of it that can be taken with you to whatever you do in the future, and just ride what could be a pretty fun wave with the right perspective. Or maybe this band will come together over it and make adjustments depending on whether it works or not. Sometimes a disaster show that you realize that you just weren't ready for is a really good wake up call. Either way I would say that you gotta do you, whatever that adds up to. They've got to do them too and if they're dead set on playing when they may not be ready that's for them to decide, but you've got to decide what is more important to you and what way makes you happier. You can plead your case again, but it sounds like you've done a lot of that so really it just comes down to a choice. There are pluses and minuses on both sides. What do YOU want to do?