Mom had a reverse mortgage that was not depleted thru the agency. How do I apply for the funds left?

There are no funds left, just the loan amount that you must repay. Typically, the person dies and the loan is then due in full, plus the interest, within 30 days or the mortgagor takes full title to the house. These companies love it when the person dies and they have loaned out less than the full amount. Personally, I would take it to my attorney to sort it out.