Thank you Amber. That is how I read the rule and how UK border guards will interpret.
Our issue is the airline check-in staff who block/delay. We asked airline for a pre-check but no luck.

Hello Just need some advice on an EEA family permit that has been granted and used by my adult step daughter for travel to UK with myself and family at Christmas. She now wishes to return unaccompanied i.e. to join me. The actual permit in her passport uses the abbreviated phrase " acc ' i.e. to ...