In Reply To :
( 1 ) No, C can't legally sell property since C has no title deed of property ,sale agreement ,affidavit,will and payment receipt does not confer title moreover C's husband who possessed GPA has also expired and without title deed C can't sell.
( 2 ) No, C can't get registered GPA ,it was B who could got registered GPA in favour of A.
( 3 ) No, since C has no title deed of questioned property so she can not deligate the powers to her son .
( 4 ) Yes, Sub Registrar has power to enquire in this regard.
( 5 ) Engage a deed writer/Advocate who do needful enquiry and inspect the record in Sub Registrar office.