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Unfortunately, no that is not how military retirement works. It has nothing to do with how long you were married or long your marriage overlapped his service.
Military retirement dies with the retiree unless they purchase the Survivor's Benefit Plan when they retire. That is the only means of having the retirement extended beyond the life of the retiree, as it creates an annuity based on your life span.
So, there is no means of filing for that after he has passed. If DFAS has said that you would not receive his retirement because he didn't sign up for the SBP at retirement, then regrettably there is no application or form that can correct that now.