Hi Jenny,
First - weather. We expected temps around 70-80 during the day and were lucky to find low 80s, so it was like a typical British summer - Tshirt and shorts weather. In the evenings the temps dropped sharply so we took sweatshirts/hoodies to maintain warmth. It wasn't too bad moving around but standing still for fireworks/parades or even going in shops (air con still on) you soon felt a chill
2) Christmas to New Year is Disneys busiest period, HOWEVER we found it no busier than our usual visits around Easter. That was until NYE when the parks were chaotic. We'd planned and were prepared for very busy so it wasn't too much of a shock. We stayed until Sat Jan 3rd and the parks were not that much quieter then.
My advice - go expecting the worst possible scenario and be pleasantly surprised when things turn out much better. We had a great time and would visit WDW between Christmas and New Year anytime.
Capt H