hi....I am going to send you a link to a photo of my kitty Sami...the red spot and hair loss sounds very much like what he suffered with...click on the link below and tell me if it looks the same.
http://www.medhelp.org/user_photos/show/274134?personal_page_id=473886
Sami also underwent many tests and everything was good....also NO mites nor ringworm. after some trial and error it was finally concluded that it was more than likely a food allergy, Chicken is usually the biggest culprit mainly because its the base of MOST cat foods and thus they are overexposed to it. My Vet suggested a 'novel' protien, meaning something he's never had before and something with NO feathers...I suggest you give that a try.
I also strongly suggest you not feed any human tuna.(even the juice)..we humans are even warned not to consume more than once per week due to the high mercury content, so its much more a danger to something as small as a cat...
Many Vets recommend NO fish at all not even pet food fish to cats....however it was necessary for me to feed Sami a diet of ocean fish along with beef for awhile since he wouldn't eat buffalo or venison as recommended. the sores went away and the hair grew back quickly after this change, I waited nearly a year before allowing him chicken again...he now has it on rotation with other proteins
you need to check the labels well b/c many foods that say beef and even some fish still have a chicken base...