Matthew knew them both. Had he meant the viginity was permanent, he would have indicated that. He would have extended the "till" beyond the birth, as he did with the field of blood. As it stands, Matthew simply noted the birth as the limit, not some future event. Matthew never noted "perpetual virginity" even though he knew them both well.
"In the economy of salvation, Mary giving birth to others would give a dubious status of Brother of God to the putative sibling;
Not in the least. Unless Mary was God's wife, which she wasn't.
it would also signal that with mothering Jesus her work was somehow not gloriously complete.
Not gloriously complete? Mary was Joseph's husband. Luke 2:48-50
When his parents saw him, they were astonished. His mother said to him, "Son, why have you treated us like this? Your father and I have been anxiously searching for you."
"Why were you searching for me?" he asked. "Didn't you know I had to be in my Father's house?" But they did not understand what he was saying to them.
"It would also conflict with her adoption of, and by, St. John (John 19).
Not really. Joseph was dead. Maybe any bros and sisters were too, or being with John was better. It really never mattered to me if He had bros, or sisters and I can't say at this point if he did. The light through the window thing, I don't believe at all.
Another argument from silence.
-A8
Two reasons. First, Matthew's focus is on the virgin birth of Christ, not on Mary and Joseph's relations. Second, the knowledge of the said relations is of intimate nature. It simply would not be known to even the closest friends of the family.
Unless Mary was God's wife, which she wasn't
In fact, the Church does give Our Lady the title Spouse of the Holy Spirit, with a good reason.
I don't understand the rest of your post, except to say that the precise physiological manner in which Jesus was born is indeed a matter of theological speculation and is not defined diogamtically by the Church.