1) Anything that fits the definition of a short barrel rifle or short barrel shotgun is subject to the NFA, regardless of when it was made. Several named exceptions have been made to this for weapons of historical interest like original luger pistols with shoulder stock holsters.
2) Anything that fits the definition of a machinegun is subject to the NFA, regardless of when it was made. They had to have been registered before 5/19/86 to be legal for non-government ownership in the USA now, even if made before 1899.
3) Yes, they are still considered weapons. State laws vary on antique and black powder firearms, but they're still weapons just like a knife or club.
4) Not sure in Florida - state laws vary on the age to purchase non-firearms which are still considered deadly weapons.
5) Anything that cannot use fixed ammunition, or was made in or before 1898 is not a 'firearm' under the definition of the Gun Control Act of 1968. So yes, an original antique is still considered an antique even if it can use fixed ammunition which is still readily available. Changing the caliber to a more modern round would change the classification, though.
6) Vehicular laws will depend on your state. They might have the same restrictions as modern firearms, or a different set - all depends on how state law defines firearms for purposes of transportation, possession, etc versus how federal law defines firearms for purposes of dealer licensing and interstate commerce.