I don't have a clue as to where the gun was made. Nor do I see anything that would suggest it being an originally Southern made gun.
I tend to agree with it being a 1850ish assemblage of parts. Assembling parts from older rifles certainly wasn't unusual, and I'm sure there were thousands made into generic inexpensive rifles near the end of the percussion era.
As for the barrel being cut back at the breech, the evidence on the gun indicates that it was, in a fairly straight forward way. As is, the current location of the rear sight is the correct location for a longrifle. If, the dovetail cut back towards the breech end was the original sight location, with the current length of the barrel, the sight would have been in an unconventional location. This is certainly not impossible, but, if you take into consideration that it looks like there is only two ramrod thimbles, and there is no forward ramrod thimble near the muzzle, plus the treatment of the muzzle cap/incised line termination on the forearm, tells me that the barrel was cut back at the breech, and that the stock was trimmed back an equal amount at the muzzle end.
Taking the barrel out of the stock would answer this question conclusively, but it actually doesn't make that much difference in the long run.
There's nothing wrong with it as it is, and was no doubt used as is. Value wise, sort of like a military rifle put together from parts from various time periods.
Have you looked back near the breech for a signature engraved on the top barrel flat?
John