I am so sorry for your losses. Nothing can be more aggravating, as we all know and have experienced.
I dont think the temp change was too fast, IME. Most lfs's dont care if their fish are spreading ich. They are in the business of selling their stock, not spending $$$ on meds or quarantining.
With my tanks I keep the temps at 80 degrees and treat the water with aquarium salt in accordance with the directions on the box. With each water change, I treat with salt but remember to only treat for the amount of water I replace. (If I pull 10 gallons of water from the tank, I treat the 10 I put back. I dont think the salt evaporates, so I dont want to get too much of a concentration.
One other recommendation: Use a quarantine tank for new fish or plants. If they survive without problems, then you can introduce them into your show tank. If the dreaded ich shows up, it wont spread to your other fish and you can treat the new fish accordingly.
Sadly, even if you do everything right you can still get ich in your tank. It is inherant with them and the bane of all aquarists. What you are trying to do is keep it from getting out of control.
I recommend keeping the water at 80 degrees (27 C) and treated with aquarium salt at all times. If you do have an outbreak it wont take as long to get the temps up and the salt acts to help kill the ich and promotes healing. I do this and have not had a problem for over a year.
Good luck!