Regarding the search for the Golden Dagger in the mausoleum, after entering and passing the fragile floor, use the Clairvoyance spell to see a pillar with a floor (which is actually inaccessible). Looking in that direction reveals a hidden door. Use the Magic Bridge spell and the Jump spell to enter.
There are some battles inside. After breaking through, you'll enter the King's Tomb, where there's a chest. If you don't use the Levitation spell, you'll fall while trying to reach the chest, triggering a battle.
The Golden Dagger is the loot inside.
I'll briefly describe the quest in the Quicksand area after crossing the fragile bridge. I'll post a map later, but since I'm using CE for fast travel, I won't be able to find the underwater section (my backpack doesn't have enough space for diving gear).
Directions are based on the map. In the upper left is a cave with a Sacred Cup and a Golden Spider encounter. Winning these battles will lead you to a contract.
To the right is a small area of Quicksand. Jump through it to get the Heart Key from the chest at the end of the passage.
In the lower left is the entrance to the city. The fork in the road next to it is a portal. Remember to open it. After entering the city, talk to the red dot in the upper right corner to advance the letter delivery quest. If you don't accept, you can simply take the portal back.
There's a girl portrait in the city; you can buy a magic stone by talking to her.
The quests in the tavern are: the cup from the cave, retrieving the golden statue from the well, defeating the Desert Lord, and finding the chastity belt key (which I didn't find).
You can retrieve the statue directly from the well, but since the wings are broken, submitting the quest immediately will result in a negative review. Repairing it before submitting will satisfy the customer and give them a resurrection potion.
Continuing to explore from the other side of the city will lead you into a circle of quicksand. Walking around inside will trigger a fight with the Desert Lord. Be careful, as you'll lose two party members immediately.
It deals significant damage and can paralyze your players, so be prepared for paralysis.
After squeezing through, return to the desert city, submit the quest, and spend the night there. The next day, a scene will begin. The old elf warrior who had been helping us mentioned that the demon is about to resurrect (but I got an error the moment I entered this scene, preventing me from progressing).