So how should we play this out? Do they confront her as she's returning? I'm not sure about following her out to the Deepsinger itself because I'm not sure how to handle that particular reveal, and I'm not sure it'd even be in visual range (or how it would react to a mermaid that isn't Alexandra coming into sight if it is).

She is probably going to be having little bouts of temper, but overall I think she's going to be in so much despair at this point that she'll be disappointed if they decide to spare her. What she probably won't do is fight them, because she knows they're the ones in the right.