I'm afraid, James, you've missed the point. Scripture is not a self-evidencing quality. If it were, there would be no need whatsoever to appeal to apostolicity, liturgical use, universal acceptance, or even consistency/rule of faith. If Scripture is a self-evidencing quality, then there would be no dispute: the Apocalypse would always already have been "in," the Shepherd of Hermas always already "out."
The only reason any particular work is called "Scripture" is because it belongs in the canon. Many Christians in the first couple of centuries of the Church thought the epistle of Barnabas was Scripture. But it's not. Why not? Because it's not in the canon. It otherwise might have passed all the tests: apostolicity, widespread acceptance (though not universal), use in the churches, and so forth. But the Church determined left it out of the canon, and because it is not in the canon it is not Scripture.
To say it another way: Scripture and canon are synonymous terms. Nothing is Scripture unless it's in the canon, and nothing is canonical that's not Scripture. You cannot have Scripture unless you have a canon. And it is absolutely clear that there was not canon until--at the earliest, if you by Trobisch's argument--the late second century, or, in the conventional view, the late fourth century.
To affirm that a document is Scripture from the moment of creation is to affirm more than is warranted. Why is it Scripture? Is it Scripture because St. Paul wrote it? If so, then we're clearly missing some "Scriptures" (Epistle to the Laodiceans). But if not, then what made some things St. Paul wrote Scripture and other things not? Was it like a switch that could be turned on and off?
If Scripture was Scripture from the moment of creation, then who was able to determine that? Did St. Paul put a stamp on his letters: Scripture, Not Scripture?
And if Scripture is Scripture from the moment of creation, then why even worry about apostolicity, ubiquity, catholicity, and all the others. All that matters is that it is Scripture from the moment of creation--it doesn't matter if it's apostolic or not. In fact, if Scripture is Scripture from the moment of creation, then why not argue that God continually adds to his Scripture? After all, if such things are self-authenticating, and I have a burning in my bosom about the Book of Mormon, wouldn't that be Scripture?
No, the "Scripture is Scripture from the moment of creation" utterly bypasses as irrelevant to itself the single most important aspect that those books which have been canonized and called Scripture have: apostolicity. This applies to the Old Testament, too. Why did the Church consider the so-called "Apocrypha" as part of the canon until the Reformation? Because the Apostles used the Septuagint and the Septuagint contains these books. The "Apocrypha" are Scripture because of apostolic usage.
The books are Scripture because they were written or authorized by apostles AND they confirm with and conform to the Apostles' teaching and manner of life.