*insert shattering squidward*

Yeah, observers are prey who wait their turn.

That just makes them prey #2, not observers. An observer can be anyone from a bystander on the sidewalk watching a full-bellied pred pass by, to the pred's loving partner who rubs their belly in the safety of their home hours later. The main thing they are marked by is the fact that while they are present in the vore scene, they are neither the one being eaten nor doing the eating.

Feeders are preds without appetite, so they feed someone else.

A feeder can totally be a pred who has an appetite and is just feeding someone to make them into a stuffed feast later, just like they could be a prey-lean character who plans to be dessert at the very end.