I have a theory that True Love was the key ingredient in the curse, not the heart of someone you love. Regina did manage to cast a curse, but maybe it was the wrong curse, or a different curse. Something obviously went wrong because people keep remembering who they are. Even before Emma came into town. The Mad Hatter, Henry, and possibly Belle. I think Regina miscast the curse.
That is a very interesting theory. He could have toyed with her and make her think killing her father was enough to enact the curse.