Your diastasis MIGHT be correctable, it just depends on the severity of your split. In any case, crunches are the worst thing you can do. It exacerbates the damage. You need to strengthen your core, predominantly your transverse. Planks, deep breathing (pull your belly to your spine) and oblique work will help.
There are workout videos to address this condition specifically. (Lose your Mummy Tummy. Julie Tupler specializes in this.)
For the skin, I'm in the same boat after 2 pregs. I'm trying agressive exfoliation to eventually get off the damaged top layer of skin, and then following up with Tummy Butter to add back in collagen and elastin. I'm only loosely hoping this will help a little. Apart from that surgery is necessary for extra skin removal.