Should something like this be considered, which I find to be an intriguing idea, I would suggest that infiltrators have the power to reduce or increase travel times IN the region they are in. Whether leaving or entering.
AKA. They and the army are in region 1, moving in region 2. The infiltrator could changing travel times in and out of region 1.
Or they could move to region 2 and change travel times going into or out of region 2 once there.
