. Most of us (me) would have just assumed that Acts was a romantic history developed from the orthodox POV.
How does Acts support the orthodox point of view? It espouses Torah observance (including sacrifices) and calls Christians Nazarenes, just like Mark, Matthew and Luke. It seems simpler to me to suppose it is a Nazarene writing, since they are the ones that orthodox writers say espoused Torah observance and accepted Paul.
I gather the idea that Acts is an orthodox writing stems in part from the fact that it was used by the orthodox, but I think this was simply because the earliest Christian writings espoused the Jewish Christian point of view and there was nothing the orthodox could do but ignore it or explain it away.
I think the orthodox point of view is a later Gentile interpretation/misunderstanding of Jewish Christian writings (e.g., James, 1 Peter, Mark, Matthew, Luke and Revelation, all of which espouse Torah observance, for which Jewish Christians were condemned by the orthodox).
It seems more complicated to suppose that a) the Marcionites were right about their version of Christianity being the original version; b) that orthodox Christians reacted to this by "Judaizing" early Christian writings at the expense of contradicting their own point of view; and c) that Jews were so wowed by these "Judaized" orthodox writings that they converted to Christianity (only to in turn be condemned as heretics for espousing the very point of view of that is in these supposedly orthodox writings).
I think it makes more sense to suppose that the orthodox and the Marcionites were later Gentile reactions to Jewish Christian writings, with the former ignoring or explaining away the Jewish Christian elements (e.g., Torah observance) and the latter removing them entirely.