I don't speak Italian but I found some connections from Courmayeur to Milano. Option with train is long, uncomfortable (4 changes) and expensive in comparison to one-change bus connection, but I can't buy tickets in advance via WWW and don't know if there will be enough seats for my group.
I don't know this country, so I'm afraid if it is wise to count on enough seats and just come to the bus at the right time… In addition, there could be some delays because of bike rides as far as I know. Are there some ways to buy tickets on the spot? Maybe ways to book? We can't be too late to Milano.