Yes, just turn up with your tickets or pass and they will log you in.
Yes. I arrived about an hour late to a reservation I made with tickets and they let me go. It's not a guided tour. I'm not sure about going on a different day, however.
You begin in the lobby where you are given a time of entrance. I guess it depends on how busy they are. We were a group of 7 and had to wait for half an hour. FREE coffee in the café while we waited and they have a great souvenir shop.
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