Maybe, though, Mary was killed just to cut off an avenue for Sam to find out how to save his brother....

Do you really think that is an option? How would YED know that Dean would eventually make the deal? Didn't he say to Dean (when the Devil's gate was opened) that he hadn't forseen Dean bringing Sammy back, but that he was grateful anyway as he wouldn't have been able to himself? Have I missed something? :o)