Thank you for the additional information.
If you vet has done no testing nor referred you to a specialist after 2 years then in all honesty it is time for a second opinion.
I would ask your vet for a referral to a dermatologist. Is a dermatologist more expensive yes but not as expensive as continuing to treat a condition with out knowing what you are treating.
The puss filed lesions make me suspect you may be dealing with a condition called Phempigus. This is a problem with the immune system that causes it to attack the bridges that hold the skin cells together. This can be idiopathic meaning no cause can be found, as a result of drug therapy including some types of antibiotics or as a result of allergies. Click here for additional information.
I would also consider getting a skin scraping done to check for mites.
Tea Tree oil can be toxic to pets so I would not use that.
Oral Benadryl can be given to help with itching. This is dosed at 1-2 mgs per pound of body weight every 8-12 hours.
Resicort which is a leave on conditioner is great for itching as well and can be used daily if needed. I have an allergy dog as well and I never run out of this.http://www.allivet.com/ResiCORT-p/26862.htm
While the Benadryl and Resicort may help I urge you to get a second opinion preferably from a Dermatologist since this is their area of specialty.
I hope this helps. Please reply back if you have additional questions or concerns and I will be happy to continue.