Hey Becky! I don't have any first-hand knowledge of online checkin but I've heard mixed things. I think you should go for it, especially since you are arriving on a Saturday afternoon. It could be crowded and if it saves you a few minutes...fabulous!! If not, no big whoop! You'll be at the Poly and hearing the music, smelling the lobby, and instantly relaxed anyways!!
Hmmm...Christmas time....well you will definitely want to see some of the resort decorations so check out the restaurants at the monorail resorts so you can enjoy all of that. Also, we actually really enjoyed our time at Trail's End over at Ft Wilderness. It's NOT fancy, by any means, but we thought it was cute and homespun. Plus Ft Wilderness is very close to WL and they have ponies that the girls could ride beforehand or you could take a boat ride. They have Mickey's BBQ over there too which I hear is nice as well. Don't forget a carriage ride! My parents did that back on our Thanksgiving trip in 2007 and really enjoyed it!
We did the CP back in 2007 (you will LOVE it, my fellow singer,....it is breathtaking).

We ate late "lunch" at the Garden Grill and saw the first show which was great because the lighting actually got more intense as the sun went down and it was really quite beautiful! Plus, you don't spend a lot of time standing in line beforehand and inadvertently hearing the entire show that is going on before the one you are there to see. (Even the line for people with packages is large and you will be standing there for a while). For that reason, we really LOVED the first CP show time. Garden Grill was great fun - some of the best character interaction we had the whole trip! I'm not sure if the characters are there now except for dinner though, so maybe that wouldn't be a good choice. Ugh....so much to think about...but so much fun!!!
Hope that helps.
Sara