I've always thought half cage for half wits. That said, if a car wants to be registered on the road these days, half cage is the way to go. Full cage is just too hard.
Yes it will be safe, however kiss goodbye to carrying anyone in the back seat. Best bet is to reregister the car as a 2 seater. Thats what I did anyway. You can't bend cages to the roofline anymore, so no head space in the back.
More than likely. However if you're going to turtle it, draw a line between the main hoop and the front of your bonnet, and make sure you head is not poking above that line, because thats where the roof will come down to. It is better however than the roof coming down to the door line.
Can't comment on this one.
Do you also sit down to piss? Really, this is not a reason to have a cage.
You cannot legally wear a harness on the road. 4/5/6 point harness (with a few very small exceptions) are not ADR approved. You would only be able to use it "offroad". Having owned registered competition cars for the last 20 years, I go out of my way to have normal belts in them now, as they're much more comfy if you have to drive more than 20 minutes somewhere.
It's safe, but I don't know why you'd do it. A good cage will do more to strengthen the car.
Cheers
Dwarf