The air suspension on the 3500s replaces nearly all of the leaf spring suspension. There is only 1 remaining spring so the suspension will not be that stiff when empty.
WD bars to more than level the load, the transfer weight to the front and axles of the trailer for better towing and steering.
The auto load leveling should apply air required to level after the WDH does it's job, similar to airbags on pickups without OEM air suspension.
I use airbags and an equalizer, they both do their purpose and work well with each other.
I would think the lower mode would be for leveling the truck, as the regular mode would have a rake. Hitches work better on a level truck, and suspensions do a little better with a bit of squat, even if it is just 1 leaf.