Why does one group or the other have to exclusively make up the leadership?
Why can't you have some leaders who are magicians, some who aren't, and the determining factor isn't whether your a magician or not, but who is actually good at leading.
The best solution here is not to form weirdly exclusive kingdoms where each group rules over themselves. It's to not have kingdoms at all, but to have democracies.
Well because it is. An easy way to tell is to simply plug in an actual race in place of the fictional races being used here.
What you said is basically like saying "Black people should never be ruled or governed by non-black people." Which is being racist in favor of black people, but that's still technically being racist.
If we're talking about the English language, then yes, anyone who uses magic is a magician
But I'm not sure that's how this SERIES uses the term. I think they use it to mean humans that can use magic.
It doesn't seem to me that Magi are even human.