Ehhhh... no... I think you're assuming too much. The internal components could be exactly the same. But we know the firmware is different. That's likely the difference between being able to install open-wrt or not. And the firmware is likely different purely because of the gimbal.
And the compass rose, etc, is likely deactivated for the P2V in the app, not via any hardware difference (probably because DJI discovered some bandwidth issues in the RE500). Earlier versions of the app did support the compass rose (and, if I remember, 640x480 resolution FPV)... so we know it's not a hardware incompatibility. Which leads me back to, perhaps, switching out the RE500 for the RE700 might free up that bandwidth problem, and again allow 640x480. But who knows. If the app is basing the availability of the 640x480 feature off of the type of camera in use, and the camera's firmware is still reporting back "hey, I'm a P2V" during the handshake, those features might still be deactivated.