1. Is there a federal regulation that prevents a smooth bore shot-handgun?
Prevents? no. Regulates and severely restricts, yes.
Under Federal law (NFA 1934) a smooth bore shot handgun = shotgun. And a shotgun with a barrel with a length below the specified legal minimum, AND an overall length below the legal minimum, so legally its a "sawed off shotgun".
That makes in an NFA item, and essentially a sawed off shotgun requires you to go through the same process as a machine gun to legally own one. (This means filling out all the needed forms, getting written approval from your local law enforcement, paying the $200 tax and months of waiting, and what ever else the Feds demand, and that's when the govt is operating at its usual efficiency, which isn't the case at the moment...)
No gun company is going to make one, simply because the market is virtually nonexistent, due to federal law. Rifling the barrel removes the gun from the sawed off shotgun class, and while detrimental to a good pattern, its necessary for any (let alone good) sales.
I've got and have used the .410 in the Contender, and also have .45 Colt and .45 ACP pistols. My experience is that while neat and useful for some things, it is inferior to the long gun shotgun for shooting .410 and also inferior to my Ruger revolvers for shooting .45 Colt.
Some of the popularity of the .45/.410 revolvers is simply curiosity. I think a lot of the remaining interest is from under-experienced people believing the .410 pistol, being a "shotgun" has the strengths and abilities of popular shotgun myths.
It doesn't. And, neither do the large bores (12,16,& 20ga) that created the reputation those myths are based on.
Pattern spread created the myth about "not needing to aim", which, of course is untrue. Effectiveness of shotguns for defense at close range is derived from the mass of shot in the larger bore sizes. It does work, quite well, within its range limits. The .410 does not share this advantage to anywhere near the same degree.
I like the .410 its a great pest gun in a long arm. In a pistol, its fun, but not as easily used for equal effectiveness. Does make a fine rat wrecker at 20-30 feet, once you understand where the donut hole is going to be.
I think most of the people buying .45/.410 revolvers are expecting something that isn't quite what they get.