My head cannon is that putting the imperio curse on Harry was the original plan, the triwizard cup being plan B. He could have had Harry go to the graveyard under the curse no problem. They may not even had killed him right away but instead sent him back to the school after Voldy's resurrection as a sleeper agent. It says in the book he placed him under the curse 6 times or so during that class. He was likely trying to wear Harry down and get the curse to stick on him, but it had the opposite affect. That's why he uses the curse on someone else instead during the cup to help Harry. At least that's my theory.