You can get the salt in the salt water fish section.
Don't worry about the substrate packing down. The hermies will re-arrange things to their liking. And once they start digging, you will have holes all over. It is funny to watch a crab walking and suddenly fall into a hole like they were not looking where they were walking.
I do use really small clay flower pots (after a salt water boil) to hold up stuff that may sink in the substrate, like the climbing wood.
Everything that I put in the tank gets a salt water wash. Shells get boiled (and I do this periodically to clean the shells between tank cleanings). Wood items get boiled. Vines get a quick dunk (as well as any plastic stuff). A quick dunk is better verses a sit in the pot boil since the plastic can and will melt.
You can get away with no lights for a while. Just turn on the lights in the room for day time, then turn them all off and try to keep the room dark during the night.