Having done many similar loops of Canada and the USA from a startpoint an hour west of Toronto I can concede that you could do it in 18 days - BUT--- you will not have very much time for visiting at any length in all the locations and with the friends you mention. Your estimate of 500mile days on many days is really going to wear on you.(Best stay off the booze at all times if you want to keep that pace.) Your distance estimate is close but seems a little short, I usually have wound up with something more than 14000km each trip, but then I did wander around. The fastest I ever managed from the Toronto area to the California border at Reno NV was 4 days of hard riding on mostly interstates and that was just to see how it was to travel that quickly and to visit the Motorcycle Grand Prix at LAguna Seca back in the '80s.
Are you sure you can't get an extra week or two so you can do it at enjoyable pace?
Another point to ponder is your start date. September can be a real mixed up bag of weather from nice warm sunny late summer -like to miserable cold rain on the high plains and the high passes of the Rockies.Luck of the draw there. If you could move the trip forward a month it might be better all over for riding.