Yeah, they can make the movie into one episode. It's just that they HAVE to do Volume 2, to explain Shamisen. The first plot episode will be Bamboo Leaf, yeah, no question. Dissapearance, in my opinion, is just as likely to follow immediately after, because it's directly related to Bamboo Leaf. So with Bamboo Leaf and Dissapearance filling up the Plot half of the season, I think Volume 2, Snow Mountain, and Endless Eight can fill up the 'filler' half. And maybe another story too.
And it all ties together to make it a pretty Yuki oriented season, as the first was with Haruhi. The third's plot, then, might focus around Volume 7, ie. Mikuru. This is just speculation, but I think it's pretty logical.