L5s1 spondy fusion candidate but high risk failure given abdominal pathologies & limitations to engage in rehab to recover" how can i get better?

Core Strengthening: Fusion surgery is a big undertaking at any age & the decision must be counterbalanced w/the risks and limitations as noted in your question.As you continue to explore this question w/your surgeon, also continue to work on core strengthening and the difficulties in engaging in rehab.That could include a walking regimen,Pilates based PT, core strengthening, lumbar stabilization &/or elliptical use.