Your spine is probably stable with a pseudoarthrosis. The pain you currently suffer from is most likely from the lack of union of this level. It is called sclerotomal pain and refers to the inside shoulder region commonly. This pain is quite commonly misdiagnosed as thoracic region generated pain when most of the time, the pain is really generated by the cervical spine.
WIthout seeing your X-rays and MRI, I can only speculate but I would generally repair this from the front. A posterior fusion is just fine but you would be in a collar for 8 weeks when an anterior fusion would have you in a collar for generally 1-2 weeks.