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...
When Jesus was found in the Temple at age twelve, the context suggests that he was the only son of Mary and Joseph. There is no hint in this episode of any other children in the family (Luke 2:41–51). Jesus grew up in Nazareth, and the people of Nazareth referred to him as "the son of Mary" (Mark 6:3), not as "a son of Mary." In fact, others in the Gospels are never referred to as Mary’s sons, not even when they are called Jesus’ "brethren." If they were in fact her sons, this would be strange usage.
Also, the attitude taken by the "brethren of the Lord" implies they are his elders. In ancient and, particularly, in Eastern societies (remember, Palestine is in Asia), older sons gave advice to younger, but younger seldom gave advice to older—it was considered disrespectful to do so. But we find Jesus’ "brethren" saying to him that Galilee was no place for him and that he should go to Judea so he could make a name for himself (John 7:3–4).
Consider what happened at the foot of the cross. When he was dying, Jesus entrusted his mother to the apostle John (John 19:26–27). The Gospels mention four of his "brethren": James, Joseph, Simon, and Jude. It is hard to imagine why Jesus would have disregarded family ties and made this provision for his mother if these four were also her sons.
As for the adelfoi / suggeneis controversy, the problem is that while the latter is a word for 'relatives' - literally meaning one of the same gens or race - adelfoi may also have a much broader meaning than 'brother'. Lot was Abraham's nephew, not his brother. And the 120 'brothers' in Acts 1 are unlikely to all be born of the same mother.
The real problem, however, is that with my Liddell & Scott and 3 years of Classical Greek I can sit here splitting reasons with you forever, but without the Teaching Office of the Church it's not persuasive. I can marshal arguments for my side and you can marshal arguments for your side, but the Truth does not depend on who is the more persuasive advocate or who has a better knowledge of Greek (admittedly I am more at home in Classical than koine). It's like two lawyers arguing -- there has to be a judge. And the traditions and teaching of the Church, for me, are the arbiter that fetches the most ingenious argument from Scripture up short like a canny old Superior Court judge, "Yes, that's all well and good, but THIS is what the law says."
Or, as an old Scots Session judge said to a plausible criminal, "Ay, that's all verra weel -- but ye'd be none the waur of a hangin'!"