First off, great job on your weight loss after having kids. I know from speaking with my patients as well as seeing my wife go through it, how difficult a task that can be. Even more frustrating when you're left with loose skin after.
I think you're a great candidate for a tummy tuck and that it would be the best option for you. It would address the skin laxity as well as tighten any diastasis (muscle spreading) that may have occurred.
In regards to your back pain, constipation, and incontinence, these all are likely related to pelvic floor issues. Going to your gynecologist to ensure there isn't anything surgical that needs to be corrected would be the first step. If not, there are regimens out there for strengthening your pelvic floor and core musculature.