Admittedly I haven't read the whole thread as I don't have time right now. But from what I have read this is what I would do if I was having the problems you are:
First: stop with the baths and treatments. If they are leaving a film on your cat, your cat is licking it up and/or it will leach into the blood stream.
Second: Just because a food is of high quality, doesn't mean your cat will not have an allergy to it. It could be an allergy to chicken protein, in which case, any food that has chicken in it will affect your cat. Try a food that has one protein in it and then do an elimination diet. Give the food a couple of days for the allergens to leave the body. Nature's Variety Instinct canned has some great flavours that have limited protein in them. Check the ingredients list on the can.
the best food is a raw diet. Maybe hard to get your cat to change but it is possible for all cats to start on one. It will give your cat a species appropriate diet that will digest as the cats digestive tract is meant to and give your cats the essential nutrients as nature intended to.
Best website for information:
Third: stop all food with fish in it. There is a correlation of a high fish diet and hyperthyroidism in cats and possibility of high mercury.
Forth: not sure if you are using any cleaner on your carpet or not, but that too can be a factor.
Good luck and I'll try to get a chance to read the thread in detail later.