Would surgery for diastasis recti still be considered cosmetic when I now have a diagnosis of Ehlers Danlos syndrome?

Under Ontario public health insurance (OHIP), each application for coverage is considered on its own merits.  you need to see a Plastic Surgeon who is willing to offer the surgery and make an application on your behalf.

I haven't been able to get even the most deformed nasty rectus diastasis covered for about three years on any patient. The problem is that although the diastasis does make your core weak, there is no true hernia, so you are not in any immediate danger of intestinal entrapment. Meanwhile, I truly believe that getting these fixed, especially when they are wide (>4cm) is incredibly important to health and well being, whether it be standing correctly, sitting up properly, not to mention avoiding problems as we age with posture and strength. The Ehlers Danlos may help, with a peer review, especially if you have a true umbilical hernia. You may benefit from mesh placement as well to try to give you better long term results- because in your case, your fascia may be inherently weak/stretchy from your collagen disorder. For the record I don't find it necessary the majority of the time.

Unfortunately, even though insurance companies say that they will pay if it is medically necessary, what they don't tell you is that, the INSURANCE companies decide what is medically necessary.  Not the doctor, not the patient, and unfortunately, not the disease.  I think you should definitely have it covered.  All I can suggest is to continue be a squeaky wheel - it helps.