I just got onto the chat on WDW's site and asked them.
Thank you for contacting Disney. Please hold while we route your chat to a Disney Cast Member who will help you with your question.
Welcome to Disney! My name is Payton. How may I help you today?
you: question about parks being at capacity
Payton: Sure! Ask away! I'm here to help!
you: if you are staying on property or have a dining reservation can you still get in if the park is "at capacity"?
Payton: That is a great question! Guests have preferential treatment on days where the theme parks are close to capacity. Resort guests will be granted access to the parks up until theme park has reached maximum capacity.
Payton: When are you planning to visit us?
you: 11/17-11/24
you: have the parks, specifically MK, ever reached capacity on thanksgiving day?
Payton: Perfect! Holidays can be a popular time for us. However, the only times that parks have reached capacity is Christmas, New Years, and sometimes the 4th of July. You would not have to worry about that for your travel dates.
you: One last question, when do the halloween decorations come down and the christmas decorations go up?
Payton: The Halloween decorations come down during the first week of November. Christmas decorations go as soon as Halloween is down. The decorations will be up in time for the start of Mickey's Very Merry Christmas Party on November 9th.
you: Ok, I lied, one more. Even though
MVMCP isn't happening while I'm there the Castle will still be decorated for Christmas right?
Payton: As I mentioned, Mickey's Very Merry Christmas Party starts November 9th, so yes the Castle will be decorated for your visit.
you: Thank you!
Hope this alleviates some stress. I know it did for me!