Just from your description I would not think that you have overdosed your tree on nitrogen. I've used Miracle Gro in the past and have never been able to burn a plant, much less kill it, using Miracle Gro even with doubled the recommended rate on much more tender plants than a dogwood. One question that I have as I have seen this happen on more than one occasion. Did you plant it too deep? A soil PH of 7 is a neutral ph. Dogwoods generally like a bit more acidic soil than that, but I wouldn't think that the ph is the culprit.