I’ll take a stab at the Dark One questions and say I agree with Pandora’s Imp: there is only one Dark One at a time. It seems pretty clear that the only way to become the Dark One is to kill the current holder of that title; hence, there can be only one at a time. And the dagger is a huge part of the Dark One’s mythology; we haven’t seen any indication that there is a dagger for either Cora or Regina, or that they even know about the dagger (if Regina did know about the dagger surely it would have been a primary quest for her to possess it so she would control Rumple. And both Zoso and Rumple were miserable with being the Dark One, but Regina and Cora relish their magic powers and seek to gain more. It seems likely to me that the ladies both sought to become magical beings, in full knowledge of what that would mean, and are happy with their choice.