The issue that I have with the inference that you are making with the inference that is being made about the potential observed rate of diet induced DCM is the assumption that the 515 case reports (numerator) in the FDA update were from the total estimated dog population of USA (87,000,000 last I looked). This is an observed rate (no upper confidence interval around this) of 1 in 169,000 dogs (> 10-6, extremely remote occurrence rate).
The issue I have with this is only a small portion of the dog population is fed grain free foods. Pedigree and Purina Dog Chow (and others) are fed to many more dogs than these more expensive, grain free kibbles. For example, in the US the number of dogs being fed Acana is < 100,000 than 87,000,000 and the reported incidence rate was 67 dogs. A more realistic rate estimate is more like 1 in 1,200 (> 10-3 or moderate risk). This is still a relatively low rate, but is much higher than what people are stating.
Yes, some reports are incomplete, confounded with genetic disposition and comorbidities, but there is an unusual, unexplained signal here. No, the exact root cause is not yet understood but until it is, people willing to continue feeding their dogs are not crazy (just believe that the benefits outweigh the risks) nor are owners who want to avoid the risk until it is better understood. This will be understood in the next year or two. There are controlled studies ongoing that may explain this.
I agree that rotating foods between differing protein sources, formulations and manufacturers is a good risk reducing practice. Supplementation as you suggest can also further reduce the risk. I fall into the camp that believe that the risks outweigh the benefits. There are 13 Australian Shepherd case reports (my dog breed) with dogs as young as 2 with DCM on grain free diets. Until the root cause is identified, I have chosen to move from my grain free food (Orijen) to a rotation of legume/tuber free formulations. If one spends some time and looks, one can find a good rotation from reputable manufacturers.