"Defeat of the constitution in the referendum by Sunnis is the biggest danger, IMHO."
There is a difference between being in the majority and being more than 2/3rds of the population in a province. If the Sunnis are more than 66% of the population in 3 or more provinces then there would be some cause for concern, and if this were the case you can be certain that AP would have gone out of its way to say so.
I would think that the majority Sunnis would be 2/3s in the Sunni triangle region, but I don't know. I hope you are right, but I'm still worried...