I'm a sheep.
A lovely, fluffy sheep.
Light player plays MF card. While defending he focuses the card to attack then receives two hits. Then LP invokes the action to return the MF to his hand and plays Han Solo, continuing to attack. Is this legal?
The unit you put into play with the Falcon's action can participate in engagements on the same turn when you use the action, yes. Just remember that, if you use the action during an engagement (which you can do, no problem), the new unit does *not* participate in the current engagement.