I don't understand why it's impossible to draw good ideas from Warlords or somewhere else. Why is moving Slow and Teleport to all schools considered a bad idea, while adding 2 control spells to Fire and Air being acceptable?

Besides, neither Slow nor Teleport is ideologically tied to Earth/Air. It's clear that it would be wrong to put Slow in Air, but here they are clearly neutral spells.

The icons for Slow/Teleport are clearly designed in the color scheme of their respective elements, so if they are moved without changing the icons, these spells will look strange and out of place "within" the frameworks of other elements (and will simply look like Air and Earth spells if you don't specifically remember that they have been changed).
The gameplay will have this side effect: each of the 4 elemental books will effectively provide full control on its own. This looks a bit strange, and it's unlikely that this is what anyone wants to achieve. In the case of adding new control spells, each book provides something unique, rather than everything at once.
Logically, only Slow is really associated with Air, while Slow and Teleport are not particularly associated with anything (especially since, in some Might and Magic games, Teleport is used in water).

In my opinion, after adding portal gates to the map, from which you can teleport to any castle without an Earth expert (and without Wisdom), control spells are a natural fit for all schools.

Rather, the opposite is true - given that the gates are designed to resemble the Teleport icon, and the icon requires a change for movement, the gates themselves may need to be changed.