Disney does not actually announce when they will shoot the Christmas Day parade until a few weeks ahead of time. Historically, it seems to have been around the first weekend in December. Let me answer some of your questions:
No...absolutely not. Not just AP's are allowed to the taping. This is just one of the marketing segments to get the word out. We signed up directly with the company who organizes the shoot:
http://lightshiptv.com/parade/
Being an extra is a lot of standing around, like being an extra for anything. It looks like there are 4 different time slots, which you will be generally coralled for a period of hours, and they will be filming individual spots and crowd scenes. Many people enjoy the opportunity, others, especially with small children, may find the waiting around hard. There is no guarantee you will ever actually be seen on Christmas Day.
The park is open as normal to guest, but certain areas may be closed or diverted to accomodate filming needs.
Areas that are being used for filming will be roped off as needed by the production crew.
Good luck.