So, I slapped the hag silly to the point where she begged for her life. Then I successfully rolled a deception check to both release Mayrina and give me power. That part worked...
Problem is, despite the sub-quest under "Cure mindflayer parasite" main quest says completed, and I also finished "save Mayrina" quest... not only is there is there a big yellow quest marker on her house (right where she stands when you first enter her cabin). Shadowheart also keeps suggesting visiting Ethel. But, she left the swamp after me getting her hair, swearing revenge and stuff.
I've noticed, for the quest to save the kid from the harpies, that in order to properly complete the quest I sometimes need to walk to a specific spot along the path from the grove to the harpy beach... is there a similar "quest trigger spot" I need to run over to stop hearing suggestions to visit Ethel and seeing her on the map quest marker?