I can respect your problem breakdown re: probabilities and likelihoods, etc. However, one would also likely agree that an attendee who transports a gun to a show, especially a loaded one, has a high likelihood of being familiar with/licensed for/cognizant of laws regarding loaded weapons, wouldn't they? If they are attending WITH A GUN, then they must not be that new to guns, would they? Just seems a little improbable to me, that's all.
With all the new people attending gun shows, I can imagine several ways that an inexperienced gun owner might show up.
One possibility might be the surviving spouse or offspring of a deceased gun owner. Perhaps, having inherited a rather bulky, older semi-auto pistol, like a Model 1911, the new owner wishes to sell or trade it in order to obtain a more manageable gun. I hope to see my grandsons someday inherit my guns. If at all possible I will be training them myself, but we don't always get everything we want.
There are probably many other ways that a person could come to own a gun with which they are unfamiliar. They may have bought the wrong thing and the dealer would not offer a refund or trade-in. I made a dealer take back a used gun I bought years ago after I determined that it was defective. You would have thought I was extracting all his teeth. He refunded but he wasn't happy about it.
Such a buyer might find themselves at a gun show with that unfamiliar and unwanted gun, hoping to get the value back.
Perhaps the person possessing the gun is a close relative of the actual owner and the owner lives in a state where that gun has been outlawed. The relative might find himself handling an unfamiliar gun in his attempt to help the owner recover the value. (This scenario is not as far-fetched as it might sound. I won't get into the details since all my guns were lost in an unfortunate boating accident.)
There are even some nefarious reasons that this could happen. Perhaps the "owner" was unfamiliar with the weapon because he stole it. He hopes to benefit from its value without ever intending to make use of the gun himself.