Do you have GmsCompatConfig installed under the Google Play Store section inside the "Apps" Graphene app?
Been a while since I've installed the Google camera app so don't remember the exact process. There was a bit of trial and error involved on my part.
I did find a lot of good info on these boards though. Try searching Google camera in these discussion boards.
I don't have other Google Play installed, and I'm using the Google camera.
There was something else I had to do to use the preview function in the camera app, but otherwise I have storage scopes set up for Google camera and can view using my gallery app.