Basically, yes, if he died before the mortgage was repaid, the Mortgagee should give the Estate time to sell the property (interest would still be accruing). The selling Solicitor would then repay the Mortgagee from the sale funds, and the Estate would repay the remainder from the Insurance Policy. If the property did not sell within a reasonable time (defined by the Mortgagee) they could then repossess and sell at a reduced price. With negative equity though this really wouldn't be to their advantage.
There is no reason why he can't insure for a larger sum to cover any contingencies, and any surplus will go to the Estate.