1 - "How does it work?" Rather well, in my opinion.
I love Emerald Aisle and can't believe the other car companies don't copy it.
2 - I've never looked for it, but I've been told by National that all rental cars are still supposed to have registration (and maybe some sort of insurance?) card in the glove box.
3 - I always ask for a copy of the rental agreement when I exit and have never had a problem receiving one.
I've had to produce the registration a few times with no problem. It was either in a pouch stuck above the driver's visor, or in the glovebox. Last time I looked at the insurance, the car was self-insured by Enterprise Holdings or whatever, and that info was with the registration. I can look tomorrow when I pick up my next car.
I very, very rarely rent anywhere else. It's just so much easier to use the aisle.