Hi there, Lesley!
Your first stop should probably be
the calendar on the Magic Kingdom Web site. That will allow you to look exactly at you itinerary in connection with the hours of the major events.
Right off the bat, I would suggest the weeks of January 13, 20, or 27. The weather in central Florida is cool and pleasant, with jackets a necessity. You might, however, find that there are days when it's warm enough to swim in the heated pool at your Disney Resort.
As for crowd levels? You're past the holiday season, of course, and Marathon Weekend has come to a close (the marathon itself is on January 13). The next major event is Presidents Day weekend, wherein you'll find that attendance spike, and the lead-in to Spring Break season. Historically, attendance is nice an low during this week.
Fireworks times are right in line with your "wishes," and you'll be able to schedule to your convenience.
January is probably my favorite month at Disney, precisely because of weather and crowd levels. It's a great time to see your favorite attractions - and work in some of those you might not get a chance to experience.
Have a wonderful time!
See you later, celebrator,
-Deborah B.