SOLUTION: I followed advice from from another thread/issue and managed to work around.
- Enable testing cheats in your game (ctrl+c, enter 'testingcheats on')
- Shift-click the affected sim and remove them from your household
- Shift-click and re-add them to the household
- Interact with Edith again, 'Attempt to Communicate'
My sim had high friendship with Edith from doing the Alice storyline previously and seemed to trigger the standard continuous-talk mode at first (or I'm just impatient) so I cancelled out and tried again. She then did her Edith-scream and the objective for Ghost Historian cleared. Friendship's still high also!
Not a full solution but it works nicely. Huge thanks to @meganspisak, worked past two issues with one fix 🙂
