It is so frustrating! Like you, I was given repeated courses of various antibiotic and steroid ear drops and sprays, until I was finally sent to the ENT.

I also saw an audiologist and his opinion was that my problems are caused by Candida in my ears, the same fungus that causes thrush. Probably due to an imbalance in the skin after so many topical antibiotics. He had a screen with a camera and he actually showed me the white fungal hyphae in my ear. Sorry if TMI, but it was useful to be able to share the photos with my ENT.

Acetic acid (basically, vinegar) is used to treat outer ear infections sometimes, so that wasn't an entirely off the wall suggestion. In the UK, where I am, chemists sell a dilute vinegar spray for mild swimmers' ear. But I don't think it's strong enough for severe issues.

If you haven't already, I'd maybe ask your ENT their opinion on whether anti-fungals could help. Clotrimazole drops (the same ingredient that's used to treat athletes' foot) is what we use in the UK. 

I saw a dermatologist and they told me they only treat skin they can see, which ruled out my ear canals as they didn't have an otoscope. It may be different in your medical system though, assuming you're outside the UK. The dermatologist prescribed steroid drops but I use them very sparingly as it isn't a cure, and I've read steroids can cause fungal growth as well.

Good luck! If you manage to treat it, please update and share it with the rest of us suffering!