Both the latter seem unlikely. The attack question is the main uncertainty. On turn 1 the River Our is bridged in several places and these cannot be blown. For the whole game. The rules say that attackers never benefit from terrain.
Attackers never benefit from terrain.
The benefits of both cards apply to all defenders as long as defenders remain under the specific terrain card. If all defenders are eliminated in the arty or armor rounds from a specific terrain type, then those effects are lost for the following rounds. The first grid location they move into counts as one move. Thus leg remain there, but mech could move one additional grid or attack. The first hit could go to an arty unit.