I'm a bit behind reading, forgive me if this is already covered (my gf took a trip and I had to wait two week to watch Lost (i.e. "True Love").
Ok, my theory... Desmond is currently remembering both timelines... unlike the previous mind-based time travel where he was confused, and had the memories mostly of one time, while appearing in the other, I think this time, he remembers both worlds, which is why he's acting all zen-like and knowing, while also going with the flow (a skeptical method to ataraxia, or tranquility of the soul".
Also: doesn't this knowing nature make him the lead candidate?
Also, it's possible, imo, that Desmond can picture more than those two moments, that he's picturing future as well as past in these time lines, and thus have a complete knowledge of how it works out, not in a single time line, but between them all.
PS: I'm also very happy the time lines are colliding (like Sun saying "it's him!" about Locke).