For the record, Bernard, I have now taken a closer look at your other two examples of Marcionite posteriority: Luke 16.17 (not one tittle) and Luke 8.19 (mother and brothers).
I find your arguments on the latter, Luke 8.19, to be completely reversible. You seem to confuse the Marcionite version
of the text with the Marcionite interpretation
of that text. I think even the Marcionite text implies, as you indeed argue, that Jesus has a mother and brothers. It may well be only Marcion who believes that the mother and brothers are not really blood relatives, not the (original author of the) Marcionite text itself. That is a crucial distinction, because it then opens up another possible line of transmission: 1. Marcion replicates a gospel text which lacks the direct statement that the mother and brother are standing outside; he himself seizes upon the slim opportunity afforded him by the fact that this information is transmitted only in dialogue, not directly by the narrator, and argues that they are not really blood relatives. 2. Later on, after Marcion has published his gospel, the editor of canonical Luke imports the directly narrated statement about the mother and brothers from the other synoptics. I wish to emphasize again, lest I be misunderstood, that I think that even Marcion's own text implies a mother and brothers; it is just that the directly narrated statement about them would make the case all the more clearly antidocetic. Also, I am not arguing (at least not yet) that this is the more probable direction of development. Pending further thought on the issue, I can hypothetically see it going in either direction: Marcion eliminating the explicit reference, or the Catholics interpolating an explicit reference.
Your arguments on the former, Luke 16.17, however, I do find to be persuasive. I would add to your analysis one more point: Marcion apparently has
"one tittle of my (Jesus') words", yet a "tittle" (Greek κεραία
) is a written mark, a stroke or a serif on certain letters. Such a term makes far more sense when applied to the law, which had been written for centuries, than it does applied to Jesus' own (as yet unwritten) words while he is still speaking them. I find, like you, in favor of Marcionite posteriority in this case.