Hi S., I am not sure if advice is what I am giving, but I am at least in touch with your emotion. During pregnancy the abdominal wall is supposed to "give and open" to allow the baby/uterus to be cradled during gestation. Although this is supposed to happen, it doesn't always go the way god intended I believe.
My abdominal wall seperated as well, but I believe if I remember correctly, after I had my twins (3rd pregnancy, all big babies, natural birth to 4 kids..what the heck is my body thinking about me?LOL!!) I had an exam post delivery and my OB told me that he believed that my abdominal wall seperated, like they should, but rather than cross ways which is I believe the "correct" direction, mine seperated long ways, like when you try to open a banana and it just splits open down from the stem. So I understand what you are going through.
I am also tall and "built", 5'8" and about 165 right now, my oldest is 14 (8lbs. 1 oz.), then the 12 yr. old (9lb. 1 oz.) and the twins are now 7 (he was 8lbs. 3 oz. and she was 6 lb. 12 oz.)so that was about 15 pounds of just baby, whew and no, no bed rest and an induction at 38 weeks I am actually made of kevlar! LOL!. I think my poor little abs said "Oh forget about it" and they gave up. I am 32 and have yet to get my tummy back like it should be, but little by little it does seem to improve. You can try cross directional crunch type exercises as well as swaying in a circular motion while sitting to work your entire abdominal region or do what I did and join Curves, which was wonderful, just ladies of all shapes and sizes and I lost over 20 inches over all, but this was also after the significant weight loss I had as well!
I am not a work-out-a-holic by any stretch of the imagination and I have lost a significant amount of weight since back in the day, about 60 pounds, so between the seperated abs and the flabby skin, I so feel your pain. Just keep at it and keep trying and maybe one day, you can get it fixed. I hope to.
I have never considered any type of vanity surgery before, well except for bringing my boobs back above sea level, they look great while I am laying down, but forget that standing up without a bra thing, LOL! However, this would be a surgery that I would seriously consider. I hate bending over in the shower to wash my feet or something and find my poor tummy dangling there so haphazardly with the big split down the center. Makes me feel bad and detest my ex-husband even more for what he did to me...cause you know it's always the ex's fault, LOL!
At any rate, keep your chin up and know that I understand and hope nothing but the best for you! Take care, Kat