At 5'7" and 140 pounds, you are probably not over-weight and after 5 pregnancies, I can safely assume your skin has be over-stretched so that its internal elasticity is minimal.
There are different types of abdominoplasties.If we assume that you had the full abdominoplasty and have a scar around the belly button, then this usually removes skin from the lower portion of your abdomen. This means that the upper abdominal skin is tightened less than the lower abdominal skin. The less aggressive the tuck, the less tight the upper abdomen although the final result result is dependent on how much laxity existed in the upper abdomen pre-op.
Also, the more the skin is "decompressed" by removing fat or by tightening the muscles, the more the skin will appear loose. Thus, if muscles were tightened alot, this might improve the contour or shape of your torso, but the overlying skin might seem looser. You have essentially proved this by sucking in your stomach and causing more, not less, skin laxity.
In addition, all bets are off when you sit since this effectively takes the skin off tension and stretch and allows its intrinsic elasticity to appear. That is why one almost always has "loose" skin and folds when sitting after a tummy tuck. Removal of these folds while sitting is not the goal or expectation of abdominoplasties.
Finally, it may or may not be true that a vertical incision is either necessary or will solve your problem.Certainly the skin tightening with just a horizontal incision is only in one dimension so the two dimensional laxity (horizontally and vertically) that normally occurs with pregnancy may need horizontal tightening. However, the same concerns may persist, just to a lesser degree and the vertical scar is usually more unsightly. Remember that some problems do not have good surgical solutions.