1) Yes, the beach is public and open to anyone (provided you pay the parking fee)
2) The Marina Entrance is closest to Millennium Force. There is a shuttle from the Breakers to this entrance, so I am guessing that there is also one from Camper Village.
3) Yes, you can buy them online at:
http://www.cedarpoint.com, Or you can buy them the day before at the park, at Ohio AAA offices, or local Tops Friendly Markets and Meijer's stores. You save $10 on weekdays, and $5 on weekends from Tops/Meijer's, and $9.75 all the time from AAA. The regular admission is $39.
Have fun!
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http://www.thepointol.com