I wouldn't add the salt into the aquarium, people have got away with it but it is hard to mix. The way that works best for me is put a small heater in a clean 5 gallon bucket with a strong power head. I let the solution run for a hour to make sure that it is at the correct temp and salinity before I add it to the tank.
As far as sand and LR
I add about 3" of water to the bottom of the tank and then the sand, it helps level out the sand and makes it easier to add water without disturbing your layout.
can be done several ways, the best way is to secure then together in a structure that you want outside of the tank. I have seen people using PVC
pipes and drilling the rocks to hold them together, I use some epoxy that is for binding rocks and frags and have had good luck with it. Just make sure that the rocks can't be pushed off by fish or urchins and break your tank.