1. If you have applied for mutation follow it up, do not worry as it is your legal right , no one can grab it, if at all some document which are fake and forged in nature which objects the mutation, approach the court with a declaration suit.
2. it is not possible to forge and grab the land due to many reasons in your case, your mother did not execute any registered documents, or Power of attorney for the same. Secondly the title is in the joint names of your parents, so either his signature or his legal heirs signature is needed to transfer his share of property.
3. Any document on a stamp paper /affidavit does give the title, so complete the mutation process.
4. who is having the possession of the land and was the mortgaged to any private individual?
6. why do you suspect of land grabbing by the said individual, did he approach you or having the possession of the land? was there any deal done with him during your parents time.
5. It seems you have taken the correct steps to mutate your property, in case of any impediment file a declaration suit in the civil court or if no issues, take over the possession and use your land, if some one is having any documents to hold the title, they will approach the court