One thing I thought of is that me, as one of the other parents, would be pretty upset if I committed to another day for this extra recital and the the girls, the very people this extra recital was scheduled around, didn't even show up. What's interesting to me was how Grandma was able to pull an extra day out of this instructor. This on top of expecting the baptism date changed makes me wonder if she's particularly good at getting people to bend to her wishes, so when someone actually says no, that it really doesn't go over very well.
I would be upset if grandma promised to deal with the driving to and from these classes and then dropped the ball on this a majority of the time, but I would probably not have allowed the classes to continue after the first few months if it was that difficult for me to deal with the driving. The OP allowed this to continue for two years. I'm wondering if Grandma would be willing to cover tuition for a dance classes (or other activity) that worked better with the OP's schedule. The only way my kids have really gotten to get involved in activities is because of the generosity of their grandparents, so I would loathe to turn anything down and try to make it work, but this whole situation sounds so complicated, it hardly seems worth the headache.
I don't think there's anything more to do at this point beyond apologizing for the mistake in scheduling and figure out how to get the girls there. Hopefully Grandma will calm down a little with some time. There are a couple of months to work with here.