Aside from the other questions...
I don't think that salt would work the same on the larvae. It works on the slugs because the slugs secrete mucus as a natural part of their movement. The salt accelerates that mucus and sucks their water out.
SHB larvae, on the other hand, have a thicker, dry skin, and whereas the salt would eventually kill them, I think the concentration would need to be so high and maintained there that it would be impractical. I'm guessing they could burrow through the highest concentration without dying. And rain, humidity etc would destroy that barrier, and the barrier would need to be highly concentrated up to 6 inches thick. At that point you'd be better off with a foot of gravel under the hive.
That's my theory, anyway. I suppose you could test it out by setting some SHB larvae( or almost any larvae about that size) in some salt and see what happens.
Some people use diatomatious earth, although that is fatal to bees too.