Well, I don't know enough to answer all your questions with certitude, but I know that for me if I drank a cup of coffee in the evening I would almost certainly grind my teeth that night. I still drink coffee (and a lot more than you) in the mornings but never after 2pm.

I will also add that coffee dehydrates you, which can also be associated with muscle tension and anxiety. So maybe make sure you're drinking enough water (and electrolytes!) as well.

I feel ya on the feel good factor though. It would take some serious side effects for me to want to quit coffee.