http://heroes.ag.ru/heroes5/news/2008/01/ - Mod for all Campaigns
http://forums.ag.ru/?board=heroes5&action=display&s=0&num=1192719850&start=125 (post 146) - bug-fixing patch
In version 3.x, spells obtained in treasure rooms, pyramids, and utopias are given in advance if the corresponding school of magic is not yet available. When the magic is learned to the required level, the spell will appear in the spellbook. In versions 1.0-2.1, as far as I remember, this wasn't the case.
If, after completing half of the map (mission), you get unwanted skills, you just have to accept it. You can't replay the mission. Maybe there is some cheat that allows you to change the heroes' progression. But I don't know of any, as I don't use any cheats at all. In the Might and Magic Campaigns, there are no such problems - almost all missions have mentors.
I have already finished playing through the original Campaigns. Now I am playing through the "Mercenaries" scenario. It's a medium-sized map, and the time it takes to complete is comparable to completing an entire Campaign due to the huge number of quests.