An MRI won't usually show any gb problems. You need an ultrasound to look for gallstones. If they show up, that's all you need. If they don't, then a HIDA scan would be called for to determine your ejection rate.
Yes, definitely it can cause the shoulder pain. I had that and thought it was from too much keyboarding, but when I went for the gallbladder removal, I was informed that was one of the typical gallbladder symptoms. As to the back in general, can't say, but it's obviously connected to the shoulder, so I guess it depends where on your back, upper or lower.