Pretty optimistic assumptions in your post- I think TOO optimistic. Read a journal called "EMERGING INFECTIOUS DISEASES" (available on the web- type that into GOOGLE).
My optimism derives from the fact that I cannot see any head of State authorizing such an attack, and I don't think freelancing terrorist groups can pull this off.
If they did use this as a weapon, and spread it widely right at the start, I would be surprised to escape with less than 25-30% fatalities.
That is your opinion, but the fact that the disease takes so long to become contagious, and the emergence of many anti viral meds in the past decade, make you scenario highly unlikely.
Another point, there was an outbreak of smallpox in New York City in 1947, in which a man arrived from Mexico sick with the diesase. The diagnosis of smallpox wasn't even made until two other infections had been identified. But despite the headstart, there were only 12 total cases.
We have the ability to keep a dangerous diesase like smallpox under control.