I love multiverse theory! I also love how a lot of people don't really understand how finite infinites work in the context of multiverse theory!

There might be a universe in which magic exists. However, there is no universe in which I exist and magic exists. That's because I was born into a mundane version of the universe, so there are infinite possibilities, but because my existence in a magical universe is 0, being accepted into a witching school is something that'll never happen for me.

So no, within the context of multiverse theory there is no universe in which multiverse theory doesn't exist, because that is a paradox and as such, has 0 chance of existing. However, it totally possible that a magical universe does exist (I would say we don't know enough about the formation of the universe to accurately judge whether or not such a universe could be possible under the right formative circumstances); it's just that the chances of any of us existing in that universe is 0.

You're getting into omniverse territory here, I think. But if accurate, then the dimensions without multiverses just lack the ability to perceive, observe, understand, measure, prove, or travel outside of their own universes. There's a whole multiverse of such isolated bubbles that will "know" that there's no multiverse, and we have a 50/50 chance of being in one.

There's a parallel universe in which the fundamental laws of physics are different: the weight of an electron, the gravitational constant, how many fundamental particles there are, the cosmological constant, ...