You can file a suit on the basis of adverse possession before a court of competent civil jurisdiction and seek ownership of the property on the basis of you being in exclusive possesion of the property for over 12 years.
Sale Deed through Power of Attorney lawfully executed in the manner recognised by law is not barred and the judgment of the Supreme Court is in fact on the different issue altogther.
A power of attorey executed before the Indian Embassy in USA in a manner recognised in law and forwarded to India and duly adjudicated and stamped can be lawfully used to transfer the flat.
No, a property can be transferred by way of a sale to any third party and also by gift deed if the property is owned by the transferor himself and from his/her own funds, to any third party.