Now I Want to know
(1) How to obtain a discharge certificate from the bank and cancel the mortgage entry in the EC
Though you people may be paying the loan repayment and have even discharged the entire loan, the bank is not obliged to give discharge certificate or NOC or hand over the title documents to a person other than the borrower until and unless authorised by the borrower or to the legal heirs of the borrower if the borrower is dead by producing the legal heirship certificate.
Therefore the mortgage entry cannot be cancelled to reflect the same in the EC without following the prescribed procedures in this regard.
(2) if this deed can be revoked by the donor and execute a fresh Gift Deed to her daughter (my wife)
The registered gift deed, which was acted upon by the donee canot be revoked by the donor at this stage or at any stage for any reason whatsoever.
(3) if at all it can be revoked by the donor, please specify the grounds/circumstances under which it can be revoked by the donor?
It cannot be revoked or cancelled by the donor for any reason or under any circumstances at this stage.
(4) whether other legal heirs claim any share in the property gifted to my wife?
If he is missing and not have been heard of for more than 7 years, then a declaration suit may be filed before a civil court to declare him civil dead, after which you can obtain the legal heirship certificate from the Tahsildar's office or revenue department and accordingly his class I legal heir, i.e., his mother can acquire his property in the capacity of legal heir and not in the capacity of donor.
No other person can claim any share in this property during his mother's lifetime in any capacity because he is unmarried person.