New roleplayers in Dwimmermount #3

I screwed up this session. Let me explain. So the group had just got back from the city where they had sold their loot and bought cool stuff. They headed back into Dwimmermount armed with the knowledge that there was a treasure vault guarded by a powerful undead and they were carrying lots of holy water and had an even picked up an additional cleric (to replace the fighter that died last session). They head over to where the deceased dwarf said the vault was located (after narrowly avoiding a poison gas trap). They picked the lock, brandished their holy symbols, got their holy waters ready, and opened the door. Then a quite a few lucky rolls happened and the undead was vanquished without ever getting to attack. Some kobolds came to investigate but they quickly retreated as the lucky rolls continued and most of their party became deceased the first round. The heroes grabbed their loot (heavily loaded) and made their way out of Dwimmermount and headed back to town. And here's where I screwed up.

Among the loot was a treasure map that was said to be worth a certain amount of gp. Since I'm giving XP for the worth of all loot taken to a safe place, I gave them the XP for all of that treasure map's gp instead of realizing that was the value of the treasure that the map pointed to and waiting to give them the XP until after they got the treasure. This brought most of them to level 4. Also, they were under the impression they would get even more XP after they got the treasure from this map so they went after it. After an encounter in the forest with some brigands they dealt with by using a sleep spell, they made their way to the Chaotic Caves (modeled after B2). Since the average level was 4, they easily dealt with the goblins and kobolds.

They then took on the gnolls and this where their luck ran out. The gnolls snuck around and attacked from behind and the magic-user was taken down to -2 hp. I had decided previously to use the mortal wounds rules from ACKS so we rolled and found she needs 2 weeks bedrest and is lame in both legs. It was getting late and everyone was tired so we called it a night. The player wasn't sure if she wanted to continue playing that character or go with her backup character. I also let everyone know afterwards that I screwed up so we'll see how it all works out.