I have tried Endonasal Therapy - like you I tried pretty much everybody I could think of in the UK but nobody had ever heard of it let alone practiced it! I tried my GP, an ENT specialist, chiropractors, naturopaths, cranial sacrotherapists......it just goes on and on.....
There is one clinic in the US that does this (in Pennsylvania) and I understand that some chiropractors do it too. The thinking is that you can get gunk building up at the ends of the tubes and it causes them to block. "Sweeping" this gunk away in theory causes the tubes to drain. However, I understand that it is unusual for people to have gunk there in the first place and a good ENT guy should be able to examine the ends of your tubes anyway with one of these long things with a light and camera on the end to check.
It is a very uncomfortable procedure (I read on the web that you can do it yourself but there is NO WAY I could) and involved the guy shoving his finger further back in your throat than you imagine it could ever go and really giving the ends of the tubes a good hammering with his finger.....not pleasant and a bit sore.
It didn't do a thing for me (no gunk) but I suppose if you do have this build up it would be an effective treatment. They also give you peripheral natural treatments designed to improve the lining of your nose but none of it helped me. He also is big on allergy testing to will offer advice about that and food intolerances etc. on the assumption that your problem is to do with allergies (which of course it may not be). Still, it was good to have tried. My ear problem turned out to be to do with a wisdom tooth.
If you are planning to get it done please PM me...you will likely have read about it on the same web site I did and will likely visit the same clinic. I can recommend places to stay, eat etc.... and can give you a complete heads up before you go.....
Have you tried a good physio? I know you're thinking "what?" but there is a large nerve that runs into your head called the Trigeminal nerve - if you have problems with posture, slouching etc. this gets trapped and can cause problems with your tubes amongst other things...also it can give you problems if you have stiff neck muscles etc. for the same reason, all according to a chiropractor and physio I saw once. Also read up on TMJ - the disorder of the jaw joint - the jaw controls the muscles that open and close the ends of your tubes and any imbalances or muscles spasms due to incorrect bite or tension in the jaw muscles apparantly can give you tube problems too.....
Please note though that I'm not medically qualified - all this is just what I have been told and from things I've read...the problems is that there can be soooo many reasons for this condition, you just really need to try stuff to see if it works - it is impossible really to diagnose since your tubes are buried in your head!