The Medallion of Greyhawk is a permanent item. The 4 blocks (cauldrons) have nothing at all to do with charges or uses. Those are more a representation of power level (specifically, the difficulty to construct).
And any treasure you earn, you get. Even in the case of a TPK (Total Party Kill). I think you even get treasure that your party earns after you die. So, if you die in the third room, but you party gets all the treasure stamps, you still get full treasure. Can someone verify that that's the way it works? I'm pretty sure, though.
Being dead at the end of the adventure only means that you don't get a survivor pin. You still get treasure. Your xp is based on the last room you were alive in, so if you get killed in the final room, you still get full xp. If you die in room 3, come back in room 5 and die again in room 6, I think you get xp for getting to room 6. Normally, there wouldn't be a gap like that (tokens that raise you have to be used in the room where you died). But there will occasionally be a way to raise a dead party member further along in the dungeon (like one year's resurrection altar puzzle). If you die in the final battle, but at least one member of your party survives, and then raises you from the dead before leaving the dungeon, then I'm pretty sure you get a survivor pin.