I'd give one feed (crumble or pellet), offer oyster shell on the side if you prefer to stick with 18% (They absolutely don't need 20 - especially if you're concerned about a fatty liver), and offer it free choice, feeding twice a day. If it lasts between feedings, then great. If not, they'll be...
Sounds to me like she's just hungry. Your bird weighs under what her breed average is, so if her keel feels fine, and there are no other symptoms, I would just stop worrying. I think you mentioned you give mash? Wetting the food actually makes them want it more. Maybe stop offering mash.
A good...