Hawkeye: How does Hawkeye’s Nemesis side scheme, Marked for Death, work?

It should say “when the scheme is defeated.”

If Mockingbird is in play, you have to put that copy under the side scheme because the unique rule prevents another copy from entering play. While Mockingbird is faceup under the side scheme she is in play, but under no player’s control. So to answer your question: you would place the core set Mockingbird that you have in play under Marked for Death, but that would still prevent you from playing the new Mockingbird until you defeat the side scheme.

-Caleb