Well, it seems you know what the weather will be and how you should pack, you just don't like the answer

.
One word: Layers.
When you head out in the am it will be cool. Those capris will be fine and you could even get away with shorts on a nice day, and a short sleeve shirt or light weight long sleeve shirt would be in order. That being said, you'll probably want to have a sweater or sweatshirt with you. In a few hours you'll shed that top layer and be comfortable. Those slip on shoes will be fine for the day. Don't forget those bathing suits. Mid 70's and sunny makes for good swimming.
Then comes the evening/nightime. You'll be looking at low to mid 50's, which isn't too cold, but without the sun if will feel downright chilly......especially while you are just sitting watching the Candlelight Processional or waiting for a parade, or enjoying yourself late at
MVMCP or late Extra Magic Hours. At a minimum you'll need that top layer you shed earlier in the day. Oh, and those toes might get a little cold at this point, too.....so socks and sneakers might be good to have. If you aren't one who tolerates the cold well you might even need a lightweight jacket.......or more. Being that we are from the northeast we always have to leave for the airport with our winter jackets and there have even been some evenings we've made use of those. Fortunately this year there are no low 40's in the forecast, but you know how forecasts can change.
So, I've told you everything you DON'T want to hear

. You need everything from bathings suits, t-shirts and sandles to jeans, sneakers and a jacket. Then, after having no choice but to bring all that you won't use half of it. Of course, if you don't bring it you'll need it.
Those are the trials and tribulations of the first week of December. Oh, we've been to WDW the week after Thanksgiving for the last four years and we'll be there a week from tomorrow.