If my registration was never used, how do I apply for a full 2-year refund? It is not possible to use the registration sticker?
You can apply for a refund of the 2-year registration fee only if the registration was truly never used and it is not more than 60 days from the date the registration was issued.  A processing fee of a dollar is subtracted from the refund amount.  To apply, you must surrender the vehicle plates and the unused registration sticker and stub at the DMV office and request a 2-year refund.  If you are eligible, the DMV will issue a receipt that displays two year refund.
What procedure to I have to follow to get this refund, though, if I have changed my name?
You must still surrender the vehicle plates at a DMV office, but request a transfer receipt, not a refund receipt.  You will then send that to the address of the Request for Refund MV-215 form along with the completed and signed Request for Refund [1] MV-215 and proof of the name change such as a marriage certificate, divorce certificate, or other court papers.   At that point, the DMV will issue your refund in your new name if you are eligible.