It's a bad idea. Water temperatures in the 70's are way to high for healthy Chinese firebellies, never mind ones that are already sick. I don't know why he suggested that, it seems crazy to me.
I wish I could be more help with your problem, but I've never had to deal with these kinds of symptoms. As you know I'd normally recommend a trip to the vets, but I really don't think you'll gain much by going to see that one.
I don't say this as a shot at you personally or anything you've done, far from it, but if anything good's going to come out of your thread, it shows people we aren't just being killjoys when we advise against buying petstore newts. From what I can see, you did everything right all the way from the beginning and set their tank up properly etc, but months down the line they are still struggling with serious health issues. even though they may have looked fine in the shop.