You are fighting conflicting constraints, though, and that's the underlying problem. Application developers won't use an IDP that protects user data. They want that data, that's why they wrote their app! Because nobody would use such an IDP (at least not without being forced to in order to be on a large platform), nobody will write such an IDP.

I'm actually working on an open-source IDP in my spare time, and to me this sounds like something to seriously consider doing right. I appreciate the idea and the discussion. I doubt anyone in the real world will care, though. (Sometimes you need to get the early adopters that do care about these things, though :)