When is Mesh or AlloDerm Necessary in Diastasis Repair?

Absolutely, NOT. The use of mesh should be a LAST recourse when the muscles (or their fascial lining) are destroyed.
I have operated on women such as yourself many times and never had to use a mesh. One, which I believe was much worse than yours, posted her pictures on this website, at http://www.realself.com/review/Full-Tummy-Tuck-w-o-lipo. See for yourself.
I would delay and consult with another Plastic surgeon (www.PlasticSurgery.org)
Good Luck.

I agree with all the other consultants that your case should be able to be repaired, using your own fascia, providing the loose abdominal skin is raised from the pubis to the rib cage (xiphisternum). Several post-op factors may contribute to failure of the repair of your diastasis recti. These include constipation, coughing, overly strenous activity undertaken too soon. As long as you have adequate time to convalesce, you should have a satisfactory convalescence and a great result! good luck.