Can you please advise whether there is anything in writing that states that a motor vehicle can only be insured by the registered owner of the vehicle or is this a myth?
It's not a myth. Most insurance companies will not allow you to insure a vehicle if it is not in your name. I believe that this practice is in place in order to avoid fraudulent claims made by non-owners of a car. You can get "non-owner" insurance to drive a car other than your own, but that does not insure the vehicle per se. It insures you and covers you for claims that might be made against you. In fact, it's a kind of liability insurance coverage.