I can just about buy Spock not realizing that Kirk would become a pawn for hardliners, as the Vulcan would probably think Kirk was already a valuable pawn for them and would not be callously expended.
I could also buy Spock knowing, from his talks with Gorkon, that sending a softer man would not result in a peace treaty. Even if Kirk wasn't the diplomat doing the negotiating, his presence would be a necessity before Gorkon's people agreed to conductive talks.
From the degree of regret Spock expresses in "Unification", though, I'm tempted to speculate that Spock fully expected Kirk to stir up trouble during the short escort mission alredy, and that the only thing that went unexpectedly there was that Kirk got caught. Or perhaps even that wasn't unexpected, considering how Spock had that box of tracer patches so conveniently at hand...