Oh boy...this could be complicated. Were either of the vets you saw specialists? If not, I always feel that when you're not getting answers, that's the time you should really consult a specialist. In Bentley's case, it sounds like maybe the place to start would be either an Internist or a dermatologist - or both. While a Neuro may be good too...I'd be wondering if this is as simple as allergies, if this were me.
Recurring ear infections, chewing paws, scratching, oily coat etc can all be symptoms of allergies. Check his tummy, arm pits, neck, inner legs for any redness/red 'spots' or brownish spots bc perhaps he has pyoderma (also can be a sympt of untreated allergies). Some allergy dogs will twitch their paws when they are really irritating them. My Marcel does that once in a while.
If the Benadryl relieves symptoms...then that also tells you something. Remember that Benadryl has a very short half-life and has to be given 3-4x a day for ongoing relief. Watch the pollens in your area at
www.pollen.com - I get daily pollen-level emails from them (I know, DORK).
Cushings also popped into my mind bc of recurrent skin and ear issues...but there would have to be other things present as well (most common are excessive drinking/peeing, pot belly, increased hunger, back leg weakness/muscle wasting, hair loss...and their are several more).
It may be wise to do a skin scraping, check for mites, fleas etcetera. Again, a vet derm would be very helpful here.
If this were me, I'd not see a generalist at this point...and I'd move on to a specialist.
If it is enviro allergies, options are OTC anti-hists, Temaril-P, Atopica, allergy shots.
Please keep us updated!