You can so long as you have their permision.
if they're on your insurance
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.
that depends on their insurance policy
Yes
Is the CAR insured? If not, it's not legal.
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.
You can register it but someone else has to drive it.
Anybody who fits the legal requirements to drive in that jurisdiction, and has permission from the vehicle's owner, can drive the vehicle.
It is illegal to drive in any US state, to drive any car, if you do not have a driving license.
Yes
The car is insured not the driver however many policies have restrictions as to who may drive the insured vehicle.
Ask someone else to drive the car while you rest in the passenger seat.