B&H info is incorrect...

If you want to see a grid in the viewfinder, you must install interchangeable screen Eg-D. (a 5 x 5 grid). There also is Eg-S which is a high precision screen that I believe is intended for especially large aperture lenses like the 50mm f/1.2 and 85mm f/1.2. I haven't used, so can't say how it works. The camera comes with Eg-A screen, which is slightly brighter than the other two.

I have a 5DII right here and it's got the Eg-D grid focus screen in it. I know because I bought the screen along with the camera and installed it myself. Somewhere I've got the "plain" Eg-A screen that came out of the camera, too.

Yes, I recall having the focus screen come loose once while out with the camera. Don't recall if it happened while shooting or just while carrying the camera. But it wasn't damaged in any way, so I just carefully put it back in place and went back to shooting.

If you want to see a grid in Live View, that can simply be turned on in the 5DII's menu. This is probably what B&H is referencing. Page 113 of the manual shows how to enable either a 2 x 2 (rule of thirds) or a 5 x 5 grid on the rear LCD during Live View.

Right. I have both the mkII and the III and I put a real focusing screen in the II but I can’t do that with the III.

While a few scratches or smudges will NOT affect the meter, the transmission level differs among screens so there is a menu choice to inform the meter which screen is in use.

The old F-1 screens (and also Nikon-F screens) were no less fragile, but being easier to handle they are far less at risk for damage.

There’s no simple way to put a focusing screen into a mkIII (but it has a grid overlay that can be enabled by menu). For perfect accurate focus I just bypass the SLR finder and focus on the sensor directly in LV.

I've changed focusing screens a large number of times on different EOS cameras including the 5D Mark II. I have used all three focus screen types Canon offers in addition to the no longer available Katz-Eye split focus screens designed specifically for manual focusing. It is a little tricky and a delicate process to insert a focus screen. They are inexpensive and once contaminated in any way they should be replaced. Those whose hands shake, even a bit, should not attempt it. I have no idea why the OP's focus screen fell out of its retaining bracket. That never happened to me. If it is not clipped in and centered properly it will be very noticeable when you try to use it.

To preclude fading into senility or worse, not keeping up will all the latest trends in photograhy, I vowed to try and learn something new every day- well- at least one a week! Tody. via the wisdom on UHH I learn something that I haven't thought of before- something unique and interesting that will serve me well. Here it is folks:

Never go outside in freezing weather and shoot the moon (eclipse, etc.) in your underwear or PJs. Besides suffering and possibly dying from hypothermia, the neighbours would probably call the police at the sight of a half-naked old geezer with a rather large and long telephoto lens. Surely that would be perceived as someone up to no-good - voyeurism- peeping-tom kinda perve thing- UGH

Oh- another thing. Never start to take your camera apart or change lenses in a big hurry- or something's gonna jam, fall off, or become damaged. If you need to shoot something or some activity that is moving quickly or changing rapidly, put your other lens on your other body- but I already know that one- Photojournalists and sports shooters don't carry multiple cameras to impress each other- there is a better reason!