I'm a San d'Orian citizen and I too was able to enter the chateau before doing my 2-2 mission.
Getting in is one thing (rank related), getting the Drain scroll quest is another (fame related). My wife and I visited the Chateau together, and while we were both able to get the General's Secret quest, she didn't have enough fame to get the Drain scroll quest or the gardender's quest.
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Samus taught us that a girl doesn't need brains to be successful. Brains are giant, evil and vulnerable to missiles.