Lol.. I like your username ^
People, you need to get more open minded. The writers could EASILY find a way to write Swanwick as MM, it's futile for us to come up with explanations, but I'm positive that if the plan was for Swanwick to be MM, that they've already got an idea of what he was doing during the invasion.
Swanwick could have read Clark or Zod's mind, and realized that the battle was very personal for Clark and Zod. Instead of facing Zod head on, J'wonnwick could have been helping rescue people around the city, keeping a low profile. Maybe J'onn wasn't ready to reveal himself to the world yet.