Brian, the thing to remember here is you are confusing Federal with state law.
Federal law considers any firearm made prior to 1898, or a replica thereof, to not be a firearm under federal law regarding purchase by a law abiding non felon that is a US citizen.
State laws can and will be more definitive, and will tend to define a black powder firearm as a legal firearm, or you would see a LOT more people carrying Colt Navy revolvers in places like California, Chicago, New York City, etc. Heck, if I had a choice between an 1858 and nothing in NYC, the big Remington would win every time! However, in more restrictive political subdivisions, that doesn't work - you cannot carry a musket on The Boston Green today without being arrested - General Gage must be grinning in his grave.
I'm just north of you in Arizona, AKA Free AZ, where any law abiding citizen can walk into a gunshop, plunk down the money, wait 5 minutes for the Brady check, or wait no time at all if they have an AZ CCW permit, walk out with gun, holster, ammo, load up, holster and conceal with no permit, license,test, government interference, etc, other than Brady of GCA '68 paperwork.
6- If is true, does not seem disturbing them?
Not one bit. I prefer it that way - criminals will get guns anyway - check all the criminal gangs in your country, with, what, ONE legal gun shop in Mexico City, and all the licensing/registering/etc., required, how many illegal guns are down there? Lots, and many of them owned by good people, like the self defense forces springing up in your border states. I wish them the best. I prefer to make it as easy as possible, and consider it a litmus test for politicians, to ensure it is as easy as possible for law abiding citizens to arm them selves, and to a level they prefer.
BTW if you fit the category of "not a prohibited possessor" you, too can carry up here. It's in AZ law how foreign nationals can carry.
I hope that clears some things up?