Of course it’s a verbal dialogue. That’s what was written down.
I don’t know how you pulled this out from Rav Shrira Gaon, unless you have a third version. He speaks of the Mishna and Talmud being written before the final radication.
You see overplaying the differences of Nuschaos between Sfarf and Ashkenaz. I never came across any significant difference. They quote each other without issue. (Tosafos references Rabbeinu Chananel and The Ramban and on references Rashi and Tosafos without issue.)
It seems to me that you are not actually coming off your learning experience but rather from some academic take on this issue. I find them very unreliable, they stick in their preconceived notions and try to defend them. They overdo the Sfard-Ashkenaz difference way overboard. Anything I’ve read on these issues from academic sources were highly unimpressive.
The Seder Hadoros describes one generation after Rav Ashi as the writers.
Being that there were mistakes, and it was up to the Rabbonim to set it straight, it is obvious that different writers and rabbis will come to different conclusions.