Well, the other way to look at this is, if Mitch were to say right now "I'm definitely going to back Rand Paul in 2016" then it looks like a political endorsement rather than him endorsing Rand because he thinks he's the best candidate the party can put forward, because at present Mitch doesn't even know who all will be in the running yet. So maybe the "almost certainly" actually adds credibility for later because he can say "I considered all the candidates on the merits, and believe Rand Paul is the best choice."
He could very easily walk this back. But he won't until he's accomplished whatever it is he's trying to accomplish with this rhetoric. And I'm not sure what that is yet.
Sounds confusing but that's how establishment, so called conservatives, speak. Its precisely why folks like Mitt Romney are ALWAYS on your side of the issue.
No. "Almost certain" means what it sounds like, IMO. You are close to certain but still not certain.
Do I trust McConnell? No, not exactly, though I do think Rand Paul can use him. But I don't see a problem with the phrase per say. Ironically enough, I'd say something similar, but from the opposite extreme.