Wouldn’t the rewards for a Roose Bolton who decisively defeated Tywin at the Greenfork, knocking the Lannisters out of the war and facilitating huge material concessions in exchange for peace, have been greater than the benefits of throwing the battle and merely weakening some rival Northern Houses? Obviously, this is before the Red Wedding and the subsequent Wardenship are possibilites he can reasonably anticipate.

I doubt it. Firstly it’s highly unlikely that Roose could have decisively defeated Tywin on his own while outnumbered and leading an army largely composed of infantry. Most likely, Roose could have managed a defensive victory that would have contributed to Robb’s larger plan – which gets you something, but not anything above the curve, as it were. 

Moreover, one of the major limits on a liege lord’s ability to reward his vassals (and thus, his ability to gain their extended service) is that pretty much all the land belongs to someone. So maybe Robb can give him some lands in the Riverlands or something, but nothing major. 

Whereas if he can massively weaken his regional rivals, Roose can potentially retake much of the traditional lands of the Red Kings.