I believe you are claiming that you think the Pastorals are a part of an original collection per Trobisch. I like a lot of what Trobisch has to say, but I don't think his assessment of the Pauline letters does a good job of dealing with Marcion.
I don't agree that the Pastorals were part of a Pauline collection at the time that Marcion put out the Apostolikon. I see those as a definite later addition. They seem distinctly post-Gospel, unlike all of the other Pauline letters. So, IMO, those are certainly a later addition, that would have been written after the Apostolikon. The Pastorals are distinctly anti-Marcionite, contain at least one reference to a Gospel, and tie in to Acts of the Apostles.
There are very few differences between the Apostolikon and the orthodox Pauline letters, with the exception of Romans. But Romans appears to have been circulating on its own as a separate open letter. An orthodox editor could have used that copy of Romans to replace the Marcionite version. The fact that Romans is at the front of the orthodox collection, but fourth in the Marcionite collection is interesting. Yes, it can be explained by length, BUT, that's only when using all 16 chapters, plus the final doxology. So what if the orthodox put Romans at the front for ideological reasons in order to point out the difference with Marcion? I find it interesting that there is no known example ordered by length that has 1 Corinthians first with a shorter version of Romans after the Corinthian epistles. In other words the biggest difference between the orthodox and Marcionite collections was Romans, and that very book is made most prominent in the orthodox collection. Maybe that's just a coincidence?
Trobisch argues that the Pauline letter collection goes back to a single source. How does this accommodate either Marcion or the Syrian collection? It seems to me that Trobish didn't take these into account.
Are you arguing that there was a single source, which branched at some point with Marcion working from one branch and the orthodox working from another branch? In you view, the Pastorals and Hebrews were both in the original trunk? So in your view the original collection had 14 letters, possibly in the order we see in Marcion. Then Marcion cherry picked from this and didn't include Hebrews or the Pastorals?
And the orthodox worked from the original trunk which included Hebrews and the Pastorals, but they decided to reorder them?
See, I think that Trobish's later work, TFEotNT, actually counters his earlier work on the Pauline letters.
In TFEotNT I think he successfully shows that a first edition of the NT was produced by a single individual in the second half of the second century. This would have been done in response to Marcionism. That work would have been where the orthodox collection of the Pauline letters, with the Pastorals and Hebrews, was established. Everything that resembles that collection traces back to the first edition of the NT.
But the Marcionite collection precedes that collection.
Also, a problem I have with Trobish's theory is that it rests so heavily on Romans 15, which I'm increasingly suspicious of. In Romans 15 Paul claims to have basically converted all of Asia Minor and Greece and claims that Christianity is already so established in Rome that there is no point in him ministering there. This is a very suspicious claim. This sounds like a claim made y Roman Christians of the second century who are trying to inflate the reputation of Roman Christianity and also make it clear that Peter is the real evangelist of Rome, not Paul.
The orthodox version of Romans has openings and closing that are very suspicious along these grounds and work to make the claim basically that Roman Christianity was well established before Paul.
8 First, I thank my God through Jesus Christ for all of you, because your faith is being reported all over the world. 9 God, whom I serve in my spirit in preaching the gospel of his Son, is my witness how constantly I remember you 10 in my prayers at all times; and I pray that now at last by God’s will the way may be opened for me to come to you.
This is not in the Marcionite version. Like the closing, this looks suspiciously like a late addition. Really, in the 50s CE the Christian faith of the Romans was reported "all over the world"? Sounds like 2nd century BS to me. Paul was remembering the Romans at all times? Sounds like anti-Marcionite propaganda.
If everything goes back to one source, then either Marcion had to have done what he was accused of - redact that source, or the orthodox version has to have built on Marcion's letters. Alternatives require that Trobish's theory falls apart at some point.