I hope Dec 12-16 are good days, we're going then. A few years ago, we were there from the 13th-20th, the 20th being a Saturday. The crowds were non-existant until the 20th, and the change on the 20th was amazing!!! We wanted to go back to a few rides in the MK, thinking it would be like the rest of the week. We waited for THREE ferries at the TTC before we got on one, and the monorail line, which would probably have been faster, was so long we didn't even consider getting in it. I will try to never be there again that close to Christmas.
As for swimming days, all the WDW pools are heated, including the ones at the water parks. It only got up into the 70's, but my sister's family from Washington State thought it was downright balmly and went to the water parks twice that week. As a Texan, I'm used to warmer weather before jumping in the pool, so my family didn't experience the water parks that trip.
It appeared to me all the Christmas decorations were in place, and if they weren't, I didn't notice. It was so beautiful, that is still one of my favorite trips.