If it were my cat, I would put 6 drops of Garlic Oil in each ear maybe twice a week and put 1 drop of pure oregano oil on the outside where you can feel the lump. The body will wash away the oregano oil and hopefully take the lump with it.
Not over feed the cat and remember, few cats in Nature reach age 5, most never see adult hood, so to have a 15 year old cat is amazing.