You would be better off picking 2 locations plus Bangkok. By catching buses and ferries between locations you lose the best part of a day. So if you move 6 times you have 8 full days and 6 partial days.
Samui and kpn are in rainy season in December. Perhaps skip them.
I am going to samui for the FMP's on the 25th and 31st. cant skip those, rain or shine.
you are right about the half days though - I shall have a "1/2 day" during the afternoons of the FMP's, a similar 2nd half in Krabi (but I am staying at Ao Nang - so Rilay, Phra Nanag will be close) and will reach Phuket from Samui at 3pm.
That last trip will be on the 1st of jan (after FMP earlier) and instead of sleeping till 3, I shall also be travelling simultaneously.
The two 1/2 days (or afternoons) in Samui will be spent doing ATV trip of Samui and bicycle trip of Phangan.

