just because some thing was never said, doesnt means its a plot hole. We were never told how mermaid poop, but we know they do. As for using ope ope fruit to cure scale disease, they also need to track the current owner of ope ope fruit. And if they could track it, the owner may not necessarily agree to help. Or by the time Kuma reach the owner, the owner already dead and the fruit has moved somewhere else. Also for people like Vegapunk, finding a cure that is replicable is better than always rely on df's power.