To open the troll door you needed to use the key you found on that left door first and in there you'd find a second key for the right door. If you used the first key you found on the right door (the one with the pedestals and potions) then there's no way to open the left door. There's only a random chest in there so nothing lost really.
To get the key in the brazier you actually have to stand on the table next to it in order to reach it. Kind of silly that a somersaulting hero that can fly in dragon form has to do that but you just can't target it properly to pick it up unless you do.
And yeah, I finished the game recently myself and I agree with you completely. One of the best RPG experiences I've ever had.
