I use same camera imager settings when working in 3d preview window and for final render. I don't see a point to have different settings when working on a scene.
Having a camera imager switch for an active camera in 3d preview window might be confusing. This gives two ways to enable or disable imager in camera object. Let's try make the workflow simpler as you noticed earlier.
Take a look how it is done in C4D with Octane plugin or Redshift. There is a global camera imager settings for all cameras or an override in active Camera object. It can't be any simpler.
We've added an option in the preferences for you to change the style of imager/postprocess panels(need to reboot blender to take effect). The "Global" style is like what you describe above.
For the multiple imagers/postprocesses, they are still supported for backward compatibility.
Cheers,
DrawFun