You can so long as you have their permision.
Yes, you can legally drive someone else's rental car as long as you are listed as an authorized driver on the rental agreement.
if they're on your insurance
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 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.
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.