"Thats fine for the feds but a state's laws can say otherwise."
None do. And endless speculative nonsense is not helping the OP solve his problem.
That's why old Willie is "cutting thru the crap"... Facts are facts and nonsense is nonsense.
Fact: You cannot give sole posession of a firearm, unsupervised, for an indefinate period of time, to another person in any state and not have it be considered a "Transfer", "Gift", "Sale", or whatever other words a statute may use to indicate a permanent change in the ownership of the firearm. Any such transfer needs to be done in accordance with State Law if between two individuals, and in accordance with applicable Federal Law at all times. No state permits a transfer to a convicted felon. The subject firearms are in the custody of the current posessor, to whom they were transferred. Any further debate is on if they were "gifted" at no cost, or if some amount of money is still demanded. Sans any written contract specifying a sum owed, the "old owner" is pretty much out of luck.
"And it's possible for the firearms to have never been transferred in any manner if they were stored by the son with the intent to retrieve them later."
He better have stored them in a locked safe then, with nobody but himself holding the combination. Otherwise they are not stored... they are transferred. You cannot give a firearm to someone to take home and keep and not have it fall under the statutes that are applicable to the jurisdiction regarding the transfer of a firearm. Have a little chat with your friendly BATFE folks and your local AG or State Police and I think you'll come to realize the error of your logic. Note that in most jurisdictions, no formal transfer is required, IE: no paperwork, receipt, etc. The mere fact that the transferer is satisfied with the legality of the transfer (same state, of age, not know to be a felon, etc) is sufficient. I've "lent" firearms to friends for testing, etc., but I knew at the moment that he was now the sole posessor, owner, transferee, or however you wish to define it. Often this is a casual "hey, shoot this for a week and let me know how you like it". That's legal IF (a+b+C) are true (same state, age, not known felon, etc). But it's not a loan. Ditto if I say "hey, I am going to the desert for a year, hold onto this for me". That's not recognized anywhere as other than a transfer and all laws apply. The "loaning" of a firearm for sporting purposes quote that you cited above is taken by the BATFE to be "under direct suopervision of the lender, and for the time period of the activity", meaning "same day, same place" for hunting.
Segue... Try "storing" your NFA things by handing them to your Mom to keep safe and sound for a year or so and try explaing to the BATFE and your local LEO's that it was not a transfer after your Mom is in handcuffs...
"you don't understand ATF regulations or Federal law any better than you do common law"
Sadly... for you... it seems that I understand the ATF regs and state law regarding firearms transfers better than you do. I am also an FFL, a collector of NFA goodies, and I have not just jumped off the apple cart. When a firearm leaves my immediate control, either as an individual or as a licensee, it's a transfer.
I fully agree that the commercial law part of dealing with the financial terms of the transaction are arguable, but not the fact that the transfer in the eyes of firearms law was complete the day he left the house and left them behind. There are really two things at play here, the OP's question revolved around the legality of how to handle it as a firearms posession and transfer issue, not one of contracts. I concede the point on the possibility of disputing the terms of the transfer but not that a transfer has been completed.