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"Pro Nunc Tunc" means that the divorce will be retroactive to prior to his death, back-dated basically.
Even though you had no assets, you may have a claim on that insurance policy. Even though he named his girlfriend as beneficiary, this is not necessarily allowed when he is married. My guess is that the insurance company is not going to pay out unless he can show he was divorced.
So, you should talk to a lawyer. You can probably get a piece (if not all) of the proceeds from that insurance policy. At the very least, I would negotiate an agreement for part of the proceeds, assuming this is what is happening here (and it sounds like it is).