So, a fellow back pain sufferer? I too feel it is probably a posture problem - you may have to try changing your posture to alleviate the pain in your upper back. My teachers all said the same to me as I too suffer from the same gripe. The advice I had was - if you are playing standing up, keep your feet apart (shoulder - width is generally advised) and point one foot slightly to the side - left is best. Then, with a mirror in front of you, check you are not leaning to one side or the other - keeping a straight back is the goal.
The achy feeling in your hands and wrists is probably a result of tension. Being relaxed when you play is vital to avoid this, but easier said than done. Make sure you are not tense when you begin to play, and if you do notice any tension creeping in - e.g. you are gripping the violin tightly- then stop, put the violin down, shake out the tension, lift your shoulders up and drop them down, flex your fingers, whatever works for you. Frequent breaks are a good idea too, especially if you are having a marathon practice session.
As for people passing comment after listening to their violin-playing neighbours, I also had no idea my neighbours could hear every note I play. Practising in my bedroom (window open), my neighbours have told me they can hear me clearly... several doors up the street !
I should charge them performance fees