Although I'm new to these forums, perhaps I can help. The darkening or the monitor occurs when the camera mechanically stops the lens down (to F8 for example) in manual mode, while the preview is with the lens wide open at F2.8. In program or aperature priority modes the camers varies the shutter speed to get you what it feels is the correct overall exposure (therefore no darkening of the monitor), but not in manual mode, where you set both shutter speed and aperature.
I am also interested in the monitor magnifier that was mentioned earlier. Does anyone have a brand name?