I'm not sure if I've done this before or not, but... welcome to the boards, Gungnir! May your enemies be drenched with sweat and shaking from fear at the mention of your name.
As for your questions, when you slew the two spirits (wizard and warrior), each one dropped a crystal. When you go down the paths, those stairs disappear... and if you look off to one side, you'll see a small holder in the wall... the ideal size to use a crystal in. Each crystal works in one room only, and you can only use one; the second won't do anything. But you can still get both jindols (the dolls on top); use crates to build a rickety ladder, and you can climb right up there.
And for the potion, you needed to wait until Mekdawa finished his comments. That's the signal that the quest is completed. If you haven't got a good save game from before this happened, you can try buying or stealing the potion back, and then giving it again. Otherwise, it may be time to hack yourself another potion and give it away, give it away, give it away now.
