I don't know much about cats eating grass, I think they eat it as digestive aid for fiber or something. When my previous cats occasionally ate it they would promptly vomit. Cats we have now are strictly indoor cats so only grass the get is licking grass clipping off shoes. They get additional fiber from eating the furniture and dust bunnies.
I understand,have to pick your battles, OtherCat knows how to play your DGF and its her cat, her call. What are the odds dry high carb diet would contribute to OtherCat getting diabetes some day.
I still hope Bj does get off insulin someday! You may want to ask the wise ones about tweaking dose to help break the spell Bj seems to be under.