"mortar and covered in salt crystals"
Perhaps this is the equivalent of using silica gel to absorb moisture.
After the burial, the little door at the inside opening was kept open by something propped underneath it and attached to a rope. Cloth wrapped bundles of salt were pushed up into the shaft and it was sealed at the outside opening. Since it was open at the tomb end of the shaft, the salt and cloth absorbed whatever moisture was left in the tomb.
After suffient time had passed and the priests felt that enough moisture had been removed from the inner chamber, they opened the outside opening, pulled the rope holding the prop from under the door at the tomb side of the shaft, effectively sealing the tomb. They then pulled out the cloth bundles filled with salt and sealed the outside opening. They needed the morter surface so it would be smooth and allow the cloth to be easily pulled, and to seal out any additional moisture from leaking in. The salt crystals were from any salt left behind, or salt which precipitated on the surface during the operation.
The alternative and Occam approved version would be that the the salt was just in the mortar and the crystals formed to the surface, like niter on old brick walls.
Was moisture a problem in the desert?
That would be my first guess.
In the "dehumidifying" version, why wouldn't they use the local desiccating agent that they used for mummifying? I want to say calcium carbonate.