It is entirely possible that the nipple is nothing but rust under there. If it breaks you can chip out the bits and replace with a modern one - get as good a thread match as you can, slot the nipple thread and it will cut a new thread as you screw it in.
WRT the barrel - while a lot of pocket pistols have a turn-off barrel, not all do. In your picture it looks too good a fit, and also note the accurate alignment of the flats. Turn off barrels are usually round so that alignment is not a problem. It is entirely possible that this was not intended to be a turn off, just the way it was manufactured. It will be threaded on to the action, but very tightly. If you do get it off then take a look at the shape of the breech. A turn off barrel has a significant constriction ahead of where where the ball would sit and it would take considerable force to push a ball through from the breech end. If approximately parallel then intended for loading via the muzzle. Also with a turn-off, the powder sits in a recess in front of the nipple (about 8 grains capacity) so there is a void there which is larger than needed for a simple pass-fire.
Dunno if you can see proof marks, but I have a similar looking one with Liege proof, and it is not a turn -off.
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