Q: If the Imperial player succeeds with "Lure of the Dark Side", does the converted leader's action card switch players also? Or does it return to the box? Maybe I missed something in the RR, but I couldn't find it.

I think taking a leader from the rebels is mean enough without taking an additional action card ;P

You definitely do not get the action card. Action cards are keyed to their faction and at no point do they change hands in the game. They also do not return to the box. They stay in the rebel player's hand.

Remember half the action cards have two leaders they can be used with. And leaders can escape.