If the mutation of land was completed in your name and entry made in ROR on 14.08.2008, the mutation done on 11.08.2017, is illegal. Based on the given facts, answering your questions:
1. You are the rightful owner of the plot.
2. & 3. A complaint may be filed against Mr. X and suit be initiated, making Mr. A & Mr. B a party.
4. Yes, you may file a complaint against the Registrar for doing wrongful mutation because the original mutation had already been done.
However, for further legal advice, I need to check the mutation papers for details.
You may reach out to me at firstname.lastname@example.org or 9818566928.
Adv. Akshay Lal
Taarkik Legal - Advocates & Consultants