If Cora is not redeemable then Regina has to be even LESS able to be redeemed because all of Regina's actions were done *WITH* her heart intact. Cora as far as we know never killed anyone until she took out her own heart. This fairytale universe/logic has heart = emotion but also has heart = conscience. So Cora did all of her actions without being able to feel or process anything as good/bad. Regina was able to and just ignored or decided she didn't care/was hurt too bad to care. If you were to use a Buffy the Vampire Slayer type logic Cora without heart = vampire... Cora with heart = vampire with a soul.