RTF is a global modification that replaces all towns in the game.

and if we're going to divide them into elements, why were darkness, light, and primordial excluded (the latter can still be excluded by incorporating it into the other six schools, but that's just my opinion)

Because these aren't elements in the generally accepted sense. And not even in the sense of Heroes 5, where only 4 classic elements are mentioned (artifacts, perks).

Fire/earth/air/water is also a return to the traditions of Heroes 3. That is, it is more universal and suitable even for those who are not interested in parts 6 and 7.

I agree with the small number of casts, so we'll have to have not 4 schools of 12, but 7 of 9

You probably meant 7 of 6.