This will come faster than you and I imagine. WSR is basically being cheated. Unfortunately if he knows, he has not shared it with his constituents who believe in the 2022 crap. The fallout from the deception will be earth shaking.
On the other hand if he opts out of the lie before 2017 then we have a scenario in which his former ally panics and gets tempted to stage a Kibaki-style heist.
I don't see Ruto going anywhere before 2017; for one, you yourself have noted that Raila and his lot largely mistreated him after the 2007/2008 elections.
2022 will be especially interesting. By then, Raila will largely be a spent force; this is his last real chance, and (looking at CORD's approach to voter registration) I think he has already blown it. And who would be the alternatives in what is now CORD? Wetangula is simply hopeless, and Kalonzo is too much of a flake.
I think Ruto will be a very strong candidate in 2022. Consider, for example, what happens if he decides that he can't trust his friends from Central and instead reaches out to what are now CORD types.