It just takes time! I even had self esteem issues and problems with intimacy with my hubby because my belly looked like a Sharpei dog! I hated it!!!
I didn't have twins but my kids are 2,4 and 6 (all by c-section). I am not a stranger to plastic surgery, having had lipo in my 20s for "saddlebag deformity" on my thighs...was the best thing I ever did. That said, I went for a consultation because of my saggy tummy and breasts. I decided against the tummy tuck because the doctor basically said that instead of the sagging, I would have a giant smiley-face scar from hip to hip (my friend has one and it is REALLY obvious and frighteningly high up).
It usually takes 3-5 years for the skin to stretch back on its own. Even though I got back down to my pre-pregnancy weight of 145 (I am 5'4") I was holding on to some weight. I joined weight watchers and lost 25 lbs and it is the weirdest thing, but my skin is not nearly as saggy as it was. Maybe it is the lenght of time...my son is 2 1/2, maybe it was the few pounds of extra weight and the slow weight loss that allowed the skin to go back slowly, but whatever, I love the results and I didn't have to pay 8-10K!
As far as the ab thing, the friend who did have the tummy tuck, did have her ab muscles sewn back together (typical during the tummy tuck). Mine have gone back together pretty well on their own. I do crunches periodically, but don't push it, you can end up really hurting yourself. Crunches are good and you can very the resistance by putting your hands above your head or crossed on your chest. These are good: the leg lifts where you lay on your back and put your hands under your buttocks flat on the floor face down, then lift your heels 2-3 inches off the floor then lift them to 5-6 inches then back down and repeat. I also make sure to do back exercises because I was getting back pain from my abs doing all of the work. If you have any other questions or need clarification, e-mail me!