It can be very cold in December all right, but it's sometimes nice and bright and sunny with it. So as long as you are wrapped up in layers it shouldn't affect your time. The Christmas season is magical...you won't even feel the cold
As for the princesses, at the moment they are available to meet on the caste stage every morning from about 10 am to 12 ish. However, the queue for this can be very long and you might spend two hours waiting to meet a princess you have little interest in. As they rotate the princesses every 20 minutes or so, you cannot guarantee who you will meet. The Auberge is quite steep alright, 48 Euro per adult and 20 per child. However, it is a somewhat better option than standing in the cold. Again, you are not guaranteed which princess you will meet, but chances are that you will see at least two over the course of your meal. Again, ensure that you request a table near the dancefloor as sometimes if you are seated at the back, you don't get to see as many. Also, it's much more beautiful to watch them dance from your table. I would say that it's likely to be very busy that week, and a relaxing lunch in the middle of the day will help you avoid the mental queues. Try to do the rides early and late and perhaps splash out on the meal if you can at all stretch to it. Check out the menu before you go though to ensure you are happy with whats on offer....it's quite limited and
as a veggie its a bit useless.