I went through a bit of Windspear Hills and collected 3 of the mask pieces, left and rested and learned new spells in the City. I came back, and when i killed my 4th Djini, i found i no longer had any of the first 3 pieces. this is either a bug, or i dropped them somewhere, but i don't think so. anyone else experienced this? either way, i can't seem to finish without, what do i do?

You don't need the mask to finish the quest, just that part of the dungeon. There may be a way to create the mask through a cheat, if not and you really want to complete it, you may have to start that part again (if you have a saved game before then)

Nah, you don't need the mask at all. Throw a cloudkill into the hallway with the invisible guardian, that'll take care of him.