I am sure others will chime in, but you seem like you did what you needed to do, but you will likely need an US based attorney to represent you. Does Grumman still exist? Just because Fidelity is acting as plan administrator doesn't absolve Grumman from responsibility. It appears you did everything that was required, but Fidelity just doesn't have the records.
You should ask Fidelity for a copy of Grumman's QDRO policy. Typically even the hint of a QDRO is enough to cause the PA to stop any payments that might be allocated to you. IOW, if the benefit is in pay status - your information to Fidelity should cause them to at least escrow "your" portion going forward until things are resolved. Also, if the participant commenced retirement payments, he was likely legally obligated to inform Fidelity/Grumman that a QDRO existed. If he didn't, you will need to sue him for your portion of the value of the payments he received. The participant might also be guilty of fraud.
If payments have not commenced, and regarding future payments if they have, you should be in a good spot, but you will need to continue to work with Fidelity and/or go to Grumman directly. They may just kick you back to Fidelity, but the squeaky wheel gets attention.