@Josephine wrote:
Evidently I took the well scene in a different light. Regina and Rumple looked to me like they were trying to capture Cora before she emerged. I think Emma and Snow show up instead and ruin their plan. It just never occurred to me that Gold and Regina were trying to hurt Emma and Snow on the promo.
I don’t think they are trying to hurt Emma and Snow at all either. That’s a really good point, they probably figured out somehow that the Well is where a portal will appear, and are trying to stop Cora before she can get through, or trap her as she comes through. That plan might involve some risk should someone else (say, Snow and Emma) come through first instead of Cora, and Henry may think they definitely will, while Rumple and Regina are willing to take that risk. It would explain that line, “You’re gonna kill them!” from Henry, and also why Regina holds him back. Did that make sense? 
And I like that idea that Lake Nostos is somehow connected to the well in Storybrooke, through the water or something. And it makes a lot of sense, since the well returned magic to Storybrooke and August said something about the water returning what has been lost, and we know that is the magical propriety of Lake Nostos.