Pretty much a novice here. It sounds like you been at biking a long time, aren't making newbie mistakes, but have run out of solutions. I agree that there is a time to seek professional help for problems. Having said that I offer two (probably lame) suggestions: (1) You asked about handlebar height, my usual riding position has my hands (on my handlebars) at about the same height as (or just a little lower than) my seat. If I have a setup with a significant drop I get discomfort in my arms and back. (2) I'm wondering if your back problem goes away when you don't bike. You might try not touching a bike for a couple of weeks. This might allow for healing/recovery to take place or you may find that your back problem is not just bike-related.