The JSON you found is all there is, and AFAIK that's the API. It's a snapshot and self documenting. It's the same data as in the table on the home page. To make the graphs I collect that data every ten minutes and save about a weeks worth.

Apple hasn't been approving wallet apps or anything that risks funds, unless maybe if you're a bank of something. You might be able to do a mining data app. I haven't looked lately at what there is in the iOS store. There are a lot of coin apps on Android.