How did this all turn out for you, Hokankai? I hope you will share an update.
Having the same troubles with my ducks. I really believe it's the ground that causes it in cases like ours. Right now, my duck yard is compacted dirt (almost like concrete, but not quite as hard). The surface is abrasive, and just rubs and scrapes the feet as they walk (like a fingernail file/emory board). I imagine that your rough gravel would have the same effect.
Stinks about the gravel, I bought some "pea" gravel that turned out to be regular gravel that was sharp rather than smooth.