Hello, welcome to the forum. To answer your questions, please refer below:
1) Give the salt, elevated temp, and water agitation a chance first. Do frequent water changes;
2) Every time you add fresh
water, add new salt. Follow directions on box for disease treatment;
3) See above;
4) a.) DO NOT ADD ANY MORE FISH. You are just compounding the issues by bringing new fish into the tank. They could come in with other diseases like fungus and internal parasites which will only compound the issues you are currently having. Wait for the tank to stabilize and all diseases to be treated before you replace any fish.
b.) Since you mixed male and female livebearers, get ready for a LOT of fry. I would consider getting a second tank for a grow-out tank for your fry, and see if you can line up some friends/family/forum members to sell/give the fry to once they're older. Alternatively, you could use the second tank to separate the males from the females, but even so you will still have some fry for awhile, as female livebearers are able to store sperm for months.
c.) Invest in a good water test kit. I use the API Master freshwater
Test Kit. Firstly, it's a liquid titration test kit, so it's much more accurate than the pet store tests, as they use the Litmus test strips and a lot of pet store employees do not have fish-specific training or expertise. Secondly, the liquid test kits last forever (200+ tests), so even though the up-front cost is prohibitive ($35, I think), it's a much better bang-for-your-buck in the long run.
d.) If you aren't doing so already, get some tap water conditioner, a.k.a. dechlorinator, and add it to the fresh water whenever doing a water change. It'll help keep your fish happy and healthy, as some water companies treat their tap water with chlorine/chloramine and it's harmful to fish.
Best of luck!