So, C6-7 has loose anterior screws due to a pseudoarthrosis, you are fused from S1 to T4 and have a new compression fracture of T4 due to the stress loading of the lower construct and osteoporosis.The picture is becoming clearer.
With a pseudo at C6-7, and a new compression fracture at T4 where the fusion ends, it looks like your best solution is to extend the fusion up to the cervical spine. This fixes T4 and C6-7, and gets rid of that stress riser. Make sure those anterior C6-7 screws are not prominent as not to irritate the esophagus.
Dr. Corenman