There is no rhyme or reason with allergies - I am allergic to things on my face that I can use on my body, and I have a longstanding patch of eczema on one elbow that is aggravated by shampoo when I wash my hair ... why not both sides?? I have a biological sciences background (pharmacy, research labs, fitness and nutrition) and I don't understand it fully.
I think the allergy tests need a blood sample sending away, so hopefully this vet could be trusted that much! It's a shame you are not having a good response, it isn't healthy (tho sometimes necessary) for an animal to be on steriods long term.
Are you able to borrow/ hire a steam cleaner and do the carpets? This should kill any dust mites, or at least knock their numbers back. Also you haven't said what you are feeding? If you have eleven it will be difficult to control what one eats I am sure. My Noah was soooooo itchy when I gave him a sample of RC, and his 'doings' are
when he has some other premium dry foods. He doesn't smell at all when he has raw meat, I was shocked at the difference in just a day or two.