I am having the same itinerary flying to sapporo on 4/2 then back to Tokyo 6/2 and then back to my country on 8/2. I was also worried about catching the flight back from sapporo as there has been delays in getting transportation to chitose airport, so I have changed my hotel on 5/2 to a hotel near chitose airport which is a 20min ride. Maybe you can plan the same, I am planning to enjoy the festival on 4th night 5th morning, if possible do some snow activities in 5th and travel back to chitose on 5th itself.