Hello,

So, if your characters find skills they normally lack the skills or stats to use, you can buff them with gear bonuses, potions, other skills, etc. in order to get them to reach this level and then teach them these skills.

They retain the skill even after they no longer meet requirements, though, so one can get and use skills significantly earlier than likely Larian desires them to get them (i.e. I taught a level 2 knight Phoenix Dive or w/e the way of the warrior 3 skill is called).

I would suggest making the spell check for the proper levels of skills and stats when attempting to use the spell, but not deleting it if it doesn't match. Just holding it in stasis until the prereqs are once again met.

Again, it's not some kind of hardcore exploit, but it is exploiting a certain system probably unintended as is.

I would be more leaning towards adding the same requirements to using the skills from your bar as learning.
Changes nothing if you have it as base anyway, but if you just used boosts or items, you need those same to actually use it in combat too.

Of course the mage follower should get his earth elemental point then, or he's quite useless

This is a problem that shows up at character creation too. Players are (currently) stuck with a set of skills they can't change, but they can reassign ability points and no longer meet the requirements for those skills. However, the game doesn't seem to object and allows these characters to be created (and those skills to be used in the game, as mentioned in the posts above).