Hey.
ad 1) I'm not a fan of having "no imager". I don't see a reason why the user should be left in limbo without knowing which settings are responsible for their outcome.
ad 2) If you want to adjust the settings of the camera imager and want to see what it does, you need to check the "Use Camera (Render Mode) Settings" checkbox in the preview and adjust your settings in the camera imager panel(s).
ad 3) When the checkbox in the camera imager is checked, it will always use the camera imager settings. No matter what is set in the preview. If the checkbox is unchecked, the camera imager will use the preview settings. In my suggestion there is no way to have the preview settings override the camera settings if the checkbox is checked. But that would in fact require additional checkboxes. But then we risk the extra confusion again.