I think that low grade fever can happen with any chronic inflammation---though have been dealing with chronic dental pain/inflammation for over a year (in a very similar story to your own) and do not run a fever....

As far as xrays go, they cannot show infection in early stages, they are also bad at seeing cracks in teeth. I have so many xrays where they found nothing wrong, but I had awful pain in that tooth. Have another situation right now as we speak. It's so disheartening and it's not in my head or your head.

Just an idea---Can you see if your dentist can make you a splint you wear at night? Just to rule out any bruxism during your sleep causing or exacerbating your pain? Just until you get more answers.