ADB, Your last line said a lot. 'Ignorance'
In the scenario you described, I do not believe the widow would face prosecution. In her case, ignorance would be an excuse to keep her out of trouble. Although she might need a lawyer and be willing to consent to a search of her home ( as opposed to a search warrant) to show good faith that there were no other contraband weapons.
You, as the current holder of the contraband, ignorance for you would probably not hold up if you were caught with it as opposed to turning it over to the authorities.
With this scenario, were it me, I would see only one choice. Hire a lawyer.
If you were just to dispose of the gun (in a river) and the events of the sale from the dealer (whether on 'paper' or not) ended up coming back to you, you would NOT be able to produce the gun. That would be a REAL problem.
I would imagine your attorney would advise you to come out with the truth of your initial ignorance of the gun itself when you bought it. That would involve the dealer (or soon to be ex- dealer) you brought it from, but it might be enough to keep you from spending more than the 20,000+ the lawyer would need up front to keep you out of jail and/or bigger fines.
Or, quickly learn to make English your second language and ignore the above
My take, JT