if they're on your insurance
Yes, you can legally drive someone else's rental car as long as you are listed as an authorized driver on the rental agreement.
Probably so, since your vehicle will likely be impounded.
Yes double the amount you were paying
No, because you were not "Driving" under the influence. You should never ride in a car or allow someone who is drunk to drive a car though.
that depends on their insurance policy
Yes
No, you cannot legally drive a rental car under someone else's name unless you are listed as an authorized driver on the rental agreement.
Is the CAR insured? If not, it's not legal.
You can so long as you have their permision.
You can register it but someone else has to drive it.
Yes, you can drive someone else's rental car as long as you are listed as an additional driver on the rental agreement and meet the rental company's requirements, such as age and driver's license validity.
Anybody who fits the legal requirements to drive in that jurisdiction, and has permission from the vehicle's owner, can drive the vehicle.