You can mix the salt into a cup of water, a bucket, the filter, whatever.. it will dissolve and spread out. The only thing you should really think hard about is when you do your water changes, make sure you only put in enough salt to replace what you took out (which is why doing 50% changes is good, it makes the math easier, hehe). Remember too, that a 5 gallon bucket only holds 4 gallon filled to those top "ridges", to get 5 gallons in there you need to fill it to the rim and its hard to carry that way Yes, lowering the tank level will help some, the splashing will increase oxygen. It might not be enough though, so watch for the fish breathing fast or gasping at the surface.
Keep an eye on your kuhli loaches though, those are cool water fish and won't like the rise in temperature. They probably already don't like 80 degrees, and if you take it higher, they'll really be unhappy (but happier than if they are dead from ich!)
