As far as I remember, this is an Android problem. If you root the phone, you can get an app that makes the SD card writable. I don’t use the Google camera, but at least Open Camera gives you the possibility to select the saving location.
Personally, I think most of the issues you raise can easily be solved by searching the forum. Most “problems” are caused by the fact the FP2 comes non-rooted by default and the new Android version behaving different.