Hi there and thanks for your useful post.

The initial history suggested something spinal in origin. Perhaps the neck or the thoracic spine (which T6 is part). The gradual weakness also implicates the spine in some way.

Has she had an Wikipedia reference-linkMRI? If so has she had an MRI of the Thoracic spine (rib cage)? It is indeed possible from an issue in that area to have an effect on the arm as well, although not so directly.

Has she had EMG tests to check the nerve connections to the arm muscles? Facet injections are for pain and do not address the weakness which for me should be the primary cause for concern.

So in my opinion, which should also be that of the treating physio, she needs MRI and EMG follow-up and asap I would say. Getting her hooked on stronger levels of pain medications is definitely not the way forward at this stage.