[Radek]

You are not right. But let us take things one by one.
1. The magic abilities. Each elite role, except a barbarian, has its own "natural magic ability" - a spell school(s) that belong to the role. It can learn spells up to and including the spell level equal

minimum(spell_school_skill, character_level)

For example, a natural magic for a paladin is Spirit Magic. A level 5 paladin with level 3 spiritcraft will be able to learn spirit spells up to level 3. If you improve his spiritcraft to 7, he will be able to learn spirit spells up to level 5. When he becomes a level 7 paladin, then (with Spiritcraft 7) he will be able to learn all spirit spells.
The spell schools that belong to a particular class are listed in your manual. Paladin - Spirit, ranger - Vine, assassin - Fiend and Moon, etc. One exception: all magic shools are natural for a warlock, not only Fiend and Moon.
Other magic schools are considered unnatural for a particular class. If the character already knows the magic school, he can continue learning that spells but only to a level:

minimum(spell_school_skill/2 rounded down, character_level)

That's why your paladin cannot learn higher level vine spells. He will be unable to learn level 7 vine spells forever. He would need vinecraft 12 to be able to learn level 6 vine spells.

As far as the Cure spell is considered, you CAN return to a previously taken role but only by means of Reset Adventures. Reset Adventures clears your role history. Make your paladin a ranger or a warlock and let him learn the spell. Reset adventures. Now, you will have a ranger or a warlock who has forgotten that he was a paladin or anything else before. Therefore, you can make him a paladin again. Well, this means that you have to begin the game from scratch but not everything is lost. Your characters will keep their money, HP, stats, mana, traits, skill levels and equipment.