[MetroMPG]

Welcome. Yes, a partial grille block will help a bit. Merc is using them on some of its latest models. (E Class, C class efficiency series).

A variable one would be best for your case, since you do a wide variety of driving. One with manually adjustable operation might be OK in your case, since I suspect you pay attention to your vehicle/driving.

Your tire question is difficult to answer because one tire may have better built in rolling resistance than the other. But all else being equal... what Daox said.