Hi, I am planning to cover most of Lijiang in early till mid of December 2014.

It is not the best timing to visit these place in Dec. but it shall be a good choice, because it is during slack season. You don't need to worry about the accommodation and transportation reservation problems.

12 days are enough to cover the three places. Here is my general suggestion for your travel plan:
KM for three days: Shilin (Stone Forest) should not be missed. West Hill and World Horticultural Expo Garden are also worthwhile. Nanping Street is the famous pedestrian in this city.

Xishuang Banna for two to three days
Lijiang and Dali for the rest days: Lijiang Ancient Town, Shuhe Ancient Town, Lugu Lake, Jade Dragon Snow Mountain and Tiger Leaping Gorge is the must-sees.

As ur travel duration is long, I would suggest you turn to a travel agency to arrange the best route for you. As well, they may offer some convenience sometimes.