We had a real struggle with this on our July-August trip down there. We ended up shipping a box of stuff home so we had room to fit our souvenirs, but we had only a small portion of our trip with really "winter" temperatures. We were trying to limit to one bag per person because of all the overage fees on the flights, with 4 people you might want to consider one extra bag to be worth it for the winter clothes.
I was also thinking about posting some things home. Glad to hear it worked out for you. We may just have to do this.
Don't forget you may think WDW in December will be warm, but that isn't necessarily the case. Our one trip in December we started with days in the 25 C and ended with a day that was only about 5 C, with some days in between that were maybe 15 C and rainy. Now, that's way warmer than what you'll likely find up north, but still not shorts and T-shirt weather.
Sounds like the temps could definitely be all over the place. I've been reading some TR's from last year and it sounded like it was quite warm for the time I will be there and only cooled off closer to Christmas. I will make sure we have a variety of gear to cover all bases
As someone who travels a lot to cold climates I'll mention a few things:
-Outerwear is pretty much infinitely rewearable, so limit as much as you can. Two scarfs and two hats per person? Why when only one will do the job? Shoes take up a lot of weight as well, so do your best to limit
I know 2 hats and 2 scarves is probably overkill. My thinking was that if the conditions are cold/wet/snowy then it would be good to be able to alternate to give them time to dry out???
And yes the shoe thing is a bother. We all have water proof hiking boots that are crazy heavy but other than on the international flights we will likely have them on our feet. Other than that we will have 1 pair of sneakers each and I guess since we have a few nicer evenings out we will need 1 pair of nicer shoes each.
-Use compression bags for those bulkier items. Those bags that you can put clothes in and roll up to squeeze out the excess air. These are great for bulky sweaters and jackets. They don't eliviate the weight limits on your suitcase, but will allow you to fit more in a smaller suitcase. Most US airlines charge a pretty hefty fee if you go overweight, which is usually 50 lbs (22 KG). Some airlines charge less if you pre-book extra bags.
We will have our trusty digital scales with us and I'm always very careful to stay underweight. I have bought some of those compression bag things. I haven't had a play with them yet but hoping they will work wonders
Others mentioned shipping stuff home/ahead, but you are traveling on so many different flights I don't think that really helps you much. However, as you buy clothes, that is an option is to ship your extra clothes home.
SP