Is it permissible to state that the mortgagee has the right to receive the full loan amount upon maturity, and not just be content with the value of the mortgaged shares?

If the mortgagee stipulates in the mortgage contract that he will receive the full loan amount when it falls due, in addition to being entitled to the value of the mortgaged shares from the company, this is usury (riba). A mortgage is a security for a debt, and the lender is only entitled to receive an equivalent amount to his loan from the value of the mortgaged item or from the mortgagor's assets. If the debt becomes due and the mortgagor fails to fulfill it, he must sell the mortgaged item to repay the debt.