Does attacking zero strength units (eg. routed units, siege engines) require a full combat resolution?
Yes, there is still combat. The rules don't specifically mention attacking into a region with only routed units. Therefore we follow the default instructions: if you march into a region with enemy units, you have a combat. As you say, it's plausible that a routed army could win (which is good; if you don't want to lose the units, you can "waste" your good cards and support orders for the turn to keep them alive"), but obviously the attacker with units contributing to battle will have an advantage.