Hi Russel,
Crystal Palace is a great place to start your day at Magic Kingdom! It's recommended that guests arrive 20 minutes prior to their reservation time. Yes, you will be able to get into the park before the official opening. When you arrive, you'll see a line designated for character dining. Simply go to a cast member and give them the name that your reservation is listed under. They'll take your tickets and let you enter the park. Nothing will be operating, but you'll be able to walk down Main Street USA to the restaurant.
I've never actually been to the parks on Halloween, but I asked my fellow Moms. They said that Halloween is just like any other magical day at the parks. However, if you go to
Mickey's Not So Scary Halloween Party that evening, that's a different story. At this event there will be characters dressed in Halloween costume, trick-or-treating, dance parties, villain appearances, and special parades/fireworks.
Hope this helps. Have a magical vacation!