1. Yes. Feed is the mechanism by which a barghest becomes a greater barghest; after that, the ability wouldn't really do anything. (I'd assume that the greater barghest can still eat corpses if it wants to, but it get no mechanical benefit from them.)
2. I'm unaware of soul-eating as a practice among barghests, but as they're evil extraplanar outsiders it wouldn't surprise me if such a thing could happen as a means of advancement. That's getting outside of hard and fast rules and into worldbuilding territory, so the GM has a lot of leeway there. Suffice it to say that in most cases one would be expected to be able to, e.g., resurrect a person who was eaten by a barghest (provided even a bit of the body remains).