And 'until' in the KJV does not necessarily imply that the negative eventually happened. When it says that "Samuel came no more to see Saul until the day of his death," it means that he never saw him again, not that he showed up the day he died.
You are partially correct. Until does not REQUIRE a change, although it strongly suggests it. Firstborn almost always applies to the first born of several. The only exception I know of is when addressing large groups of people (Exodus), in which case firstborn can mean the first and only child in some families.
If you know of others examples, please share so I can learn.
The plain meaning of those passages is that Mary had other children. If all those passages together mean otherwise, then the Holy Spirit was rather imprecise in His writing...