The Effect /validity of a Power of attorney is till the death or during the lifetime of teh person who issue it, irrespective of the clauses in it.By the laws , a power of attorney expires on the death of the principal. It does not make any difference whether the POA is registered or unregistered, if the sale agreement execution date is after the death of the maker of the POA(the deceased)
2. In your case the Power of attorney cannot execute any document on behalf of any other person, wife/ relative(legal heirs of the deceased).
3. The registration has to be done by the legal heirs, i.e, the wife and children and mother if alive.