I feel your pain, you just want to ride and can't because of an annoying pain. I have a big head (no comments please
) and have never found a helmet that really fits. Shoei, HJC and now finally a BMW Sytem7 Carbon size 64-65. It's taken a while and several hundred dollars but I think the BMW helmet is the one for me. The Full Face part of the helmet is easily removed and this is how I ride most of the time. Reduced protection aside, without the face portion, the helmet is incredibly light. After 10,000 miles breaking it in, it now fits comfortably, allowing 1,000 mile 18hr rides in one go.
I've also had the BMW comms system fitted. Far superior to the Sena Evo I had on my HJC, at least for me. But even this caused discomfort. The bracket for the mic protruded from the helmet liner and caused severe pain after an hour or so. Finally my very good dealer took a day or so to fettle the fitting of the mic into a different position. Now there is no pain.
Headache wise, since I changed out my stock screen for an XXL Extra Wide CalSci headaches have disappeared. I've put this down to a lack of head buffeting turbulence and noise from the CalSci. I sit very tall on an RDL, 2" higher than a BMW extra high seat and am 6'5".
All this being said I still occasionally get a helmet pain spot. There's no pattern to it, for instance I rode from London to Graz in one day, 1,600kms and was comfortable all the way. Then rode 200kms a few days later and got a pain spot. Removing the helmet having a scratch and going again often solves the issue. No idea why.
In a few days I'm off to the northern tip of Norway, planning a 1.600+km leg to complete the Iron Butt SS1600kms. Hopefully all will go well across the Autobahns of Germany etc. In the meantime good luck with the helmet hunt.
Safe riding and have fun,