You've had every test I would have recommended, so I'm as baffled as you are. Since this pain is located in your thoracic spine, encompasses your chest area (ribs and sternum) and makes you feel as if you cannot fully expand your lungs, which also produces more back pain, I think I would suggest you see a pulmonologist. It's definitely the next step I would take.
I hope you find the answer soon as I can only imagine how difficult it would be to feel like this and also take care of two children.
I hope you will let us know what you find out.
Best of luck,
RubyWitch