Neil,
You acknowledge MAYBE, maybe I might be right. Fine. I will start with that. Now on to some of your specific points.
Why would Marcionism attest to popular messianic expectations rather than simply widespread doctrine in the "judaisms" of the day -- that is the day of Marcionism -- post Second Temple era.
Let's start with asking the question when did the Marcionites say 'Marcionism' (that is Paulism or Christianity or whatever they called themselves) originate. The answer first century CE. I don't know whether it was before or after the destruction of the temple. But let's say 'first century CE.' I am not sure that at the time of the temple there were 'Judaisms' in Jerusalem. There was the idea that Jews were at war with the Roman state and the captives in the city were either with or against the revolutionaries.
I think this is proper context for Marcionism. Whether or not we accept the Marcionite claim that they were a first century phenomenon they thought of themselves as a first century phenomenon (like the orthodox) and specifically were a reaction against 'the Jews' and their belief that the Law and the prophets pointed to the advent of a Davidic messiah. I don't remember offhand what the evidence is on behalf of the idea that the Jewish messiah from the Scriptures
hadn't arrived yet. Murky stuff. But clearly:
1. 'the Jews' believed that the son of David, the messiah would arrive and this was revealed from the Law and the prophets.
2. the Marcionites apparently accepted the Jewish claim that the Law and the prophets announced such a figure. Apparently 'the gospel' (the Marcionite) gospel knew this (i.e. that the Law and the prophets pointed to the coming of a son of David, messiah and that the Jews 'knew' this and that original evangelist Paul 'knew' that the Law and the prophets knew this and the Jews knew this).
3. Jesus represented something other than the revelation or secret knowledge of the coming of a son of David messiah
We don't need to consider (3) for the moment. But clearly Marcionism begins with a monolithic concept of 'Judaism' rather than Judaisms which I would argue could only have emerged during the War or in a later period like that associated with Bar Kochba.
Could Marcionism have been developed as a reaction to the messianic expectations aroused by the second century figure Bar Kochba? The consensus of scholarship is that Bar Kochba did not claim to have Davidic lineage. So it would seem that the Marcionite model for Judaisms belief that the messiah the son of David was rightly announced by the Law and the prophets does not seem to fit.
In other words, Marcionism either reflect a monolithic Judaism that emerged after the bar Kochba revolt or before the revolt but not during the revolt.
Since however the (Marcionite) gospel is focused on the prediction from Jesus that the temple would be destroyed it seems unlikely that the Marcionite understanding that the Jews believed the Law and the prophets predicted the coming of a Davidic messiah and Jesus wasn't that guy was associated with any other period other than that associated with the first Jewish revolt. So with respect to Andrew's follow up question - it is possible to consider that the Jewish belief in the Law and the prophets predicted the coming of a messianic son of David might have suddenly arisen in the period immediately following the first revolt, the (Marcionite) gospel presents that belief as being pre-existent and reflective of the period before the revolt. In other words, 'Jews' believed that the Law and prophets predicted the coming of the messianic son of David.
“Finally, from so little sleeping and so much reading, his brain dried up and he went completely out of his mind.”
― Miguel de Cervantes Saavedra, Don Quixote