It's not a given that all four schools will be combat-oriented, and only combat-oriented. So, it's quite normal that they were divided this way, considering these elemental dragons of yours.

What role did you play? Connected campaigns are only in The Shadow of Death. In the other two parts, the campaigns are separate stories, just like in the original Heroes of Might and Magic 4.

In The Restoration of Erathia. Maybe I'll try the other parts later. I only noticed the gameplay. And the monotonous campaigns.

In my opinion, seven schools of magic is too much. And the dragon's point of view doesn't quite fit here – in Heroes of Might and Magic 5, in Ashan, there are only four schools of magic :).

We'll have to see how diverse the implementation will be. Indeed, how many different types of spells can you come up with? Although the Dark Magic from Heroes of Might and Magic 5 is a good example.
In my opinion, the possible maximum number of combat spells is 20 (i.e., one spell for each element for each magic circle). But, because there is one school of magic for each element, there will be more. Oh well. We'll see :).