I know it starts at 7pm but how do we know people who havent purchased tickets arent still in the park waiting to go to all these events? Whats to stop anyone else that has regular admission from just hanging out at the magic kingdom after 7 ?
Others have covered this one pretty well. You get a separate (not your MagicBand) wristband, which you are required to show typically to go on rides, or M&Gs, or basically just to any cast member that asks. If you do not have a band, you are asked to leave.
If i buy tickets to mickeys christmas party do i need regular park admission?
As others above have said, the MVMCP event is a separate ticket. The party starts at 7pm, but your ticket to this party only gets you in after 4pm on the day of the party. So, if you have tickets to MVMCP, you cannot enter the park before 4pm without a separate admission ticket.
When this party is happening are the rides open?
This page here on the main site has all the info you need about what rides, meet and greets, shows, etc are available during the party
http://www.wdwmagic.com/events/mick...-2014-mickey's-very-merry-christmas-party.htm
How many tickets do they usually sell for this party?
I've seen estimates that they limit it to 20,000 tickets. I've never been on a sell-out night, but the nights I've been the crowds feel very low. Rides are typically walk-on. The parades / fireworks are pretty busy though, and that leads to your next question.
Will lines be outrageous for character meet ?
The meet and greets have the longest lines of anything during the parties. I've yet to stand in line for one, so I cannot give you an estimate (though a friend just told me they waited 45 minutes for Anna and Elsa, which is long, but not as long as typical standby).
I dont understand how they can tell who has tickets to this and who just stuck around after the park closes
Lots of cast members looking at wristbands.