Thanks for your comment.
Actually in my (post submission) playtest, since the Perception check's DC is quite low most of the PCs found the secret door after breaking the illusion and didn't go to the false tomb, so maybe there is something to change here.
Anyway I made the mummy ambush them during the final fight, attacking the PCs at the back of the melee, it was quite fun (especially since the other monsters have low durability).
The backstory indeed needs tinkering, I had to cut what I initially wrote to save space but in the end it makes the dungeon more adaptable.
Thanks for taking the time to read it !