Tim, I'll try to answer your questions the best I can. With a camera with a FOV (Field of view) of say 135d (common to many rear view cameras, objects (mainly other cars) appear so quickly that it rarely gives you the reaction time you need, lets realize one thing most rearview setups are intended for backing up with a boat or trailer, or to help out in parking situations, not at speed to make lane changes, merging, etc. Especially here in the States, driving a RHD car, with no rear visibility in traffic the camera is almost a must (combined with the side mirrors) I find that when another car is completely framed in the monitor I can change lanes in front of him, if not, I dont, seems like a simple formula for me.
Your question of multiple cameras seems feesible with perhaps the issue of where to mount them and not disturb the appearance to the car, the multiple imputs are not a problem when using a video switcher, most of these are geared toward a back up camera, and a GPS system, so when you engage reverse the circuit to your reverse lights provides the power to kick in the second source (the camera). By using a pressure switch you could put right where you wanted it and it woulld be a manual action.
The question of one or multiple cameras seems to be best answered by envisioning a 90d cone with the camera at the small end if you will, now drawing out from your camera position to see what your coverege will be, if a car is at your right or left rear corner you will probable see its back end and if following the rule of having it fully framed, you would stay put either letting it by or speeding up, framing it, and changing the lane. For me this is sufficent, I for one wouldn't want to paste cameras all over my car, that suction cup thing while probably great for the occational track event is meant as a temperary mount, we are looking to permanantly install ours.
The truth of the matter is that the monitor is the weakest link here, the camera is capable of much better resolution than the monitor is, with a total vertical res of 234 pixels, thats less than the NTSC standard (which is not the greatest to begin with), so you can see having a 16:9 monitor with this 234 pixel vertical count is far less desirable than a 4:3 monitor of the same. LCD also ads inherant problems, one is substandard off axis viewing, two is motion artifacts and three the lack of contrast (or the ability to make black) as well as the wash out factor, even though most are coated for anti-glare, just try to see it with sunlight on it, it will totally wash out, so position of the monitor becomes crucial. Lets face it the monitors we see in these applicationa are neat and small, but are also at the absolute bottom of the food chain as far as video displays are concerned (mostly made in China) their real market is for the bling, bling muliple DVD display/car dwelling video game, spinny wheel Hummer crowd where they sell real well. That is not my thing and thank goodness I dont have to deal with that.
In conclusion, we can adapt these little camera/monitor combinations to suit our needs for a safer ride if we pick the correct camera, monitor, mounting system as well as placement of the components for a usefull and cool addition. Hope this was helpful and answered your questions.
Chris