I have recently started using the poultry cell (on occasion, need indicated by hatch result in Silkies despite no excess treats) instead of PND... I think they do have different purposes.
Feathers actually only contain about 2% digestible protein in the form of keratin... Often it can be boredom or just habit even. Maybe adding in some enrichment (for the mind) can help to stave of the feather eating? But it's true that new feathers are made from 90% protein and it's amino acids. And I saw that some birds who do feather pull and eat are actually going after the bloody or meaty bases of the fresh plucked feathers.
I remember reading that oil actually wasn't that helpful in clearing crop issues and massage was best... but knowing where the blockage is can be difficult.. This is one area I have MUCH leaning to do still and haven't faced it yet. Maybe getting some extra grit in can help?
I HATE to mention it... sometimes slow crop can be an indicator of Marek's, and molting could make other things rear their ugly head.
Hoping for quick recovery!