Hi Richard.
I agree with all your assessments and comments above. As you mention, the restriction in the bore was meant to keep the ball from compressing against the powder. While the breech designs do seem to vary, they all seem designed to handle heavy charges of black powder. The barrel walls at the breech are very thick.
Thanks for posting pics of your Matchlock. Yes, that would be a tough job with the bore not having a removeable breech plug. But the gun definately looks restorable. There appears to be a missing piece of wood behind the breech area of the stock. From the photos, it doesn't look broke. Just missing. Originally, maybe a different colored piece of wood, maybe with a horn cap ? Or maybe one piece of elaphant ivory somebody removed
But I don't notice any holes in the bottom section where nails or pins might have been (?). Anyway, be careful with that forearm. Sounds like it might be delicate.
Rick.