You can't successfully heli-coil the crank, anyway... There's not enough material around the eye to support the threads. Same reason you can't ream/tap them to the larger 9/16 size, even though it's physically possible to do so.

FWIW, one piece cranks are cheap now...a full setup of bearings, cups, cranks, and hardware would run roughly $25.

The correct move is still to return it, though. If you bought it assembled, it's their responsibility.