1) Each item has an 'item power', the sum of this field for equipped item can not be over your level. To see what item power you can use, try the 'skills' command, it'll display 'your equipped item power is x out of y'. It'll also obviously display the current sum of equipped items' item power. So you'll be able to apply an item if its item power + your current item power is < your max item power.
2) Some spells are reserved for followers of certain gods. Any spell you can find on a book can be learned. You can learn all skills except meditation and levitation (i think), which are special skills for monks resp. dragon (maybe other class?)
3) For a dragon yes. Now if you have an item with attuned path and another item with another path, i _think_ that it'll cumulate, too.
4) Improvement potions are interesting over level 10, iirc. They will increase max hp / max gr / max sp, up to a certain value.
Each level has a multiplier/random value associated for hp/gr/sp, improvement potion will maxout that value. Drink as many as you can, till you are over level 10 (iirc) AND the game says 'you are already perfect'
5) Some spells, apparently mostly summoning spells, now take time after casting. It's a pain, not sure how it was decided / which spells exactly have that delay (died also because of too many earth walls, once). Not sure either on how to improve that.
Hope this helps
