1. You have to file a civil suit for cancellation of the registered sell deed, also case of cheating and fraud has to be registered on the POA holder.
2. You can file a single case against all 5 persons and 5 registered sell deed as cause of action is same and the similar relief claimed.
3.You have to make X and the buyer the party if uncle had any role then he is also a necessary party.
4. yes.
5. Check if the property is mutated in the buyers name then give an objection before the revenue authorities.