thanks for your comment on the translation of Marcion's opening, and I also went back to your blog to read, again, through all the fascinating discussion related to your theory about Bethsaida asf. I think, we simply have to acknowledge that from what I have seen so far, the beginning of Marcion's Gospel is one of, if not the most difficult part with regards to a potential reconstruction. If the rest were of the same complexity, one would better give up. Now, from reading on in the text, it becomes slightly clearer what might (or might not) have been part of the opening. I have long thought about the Isa. 61:1 quote, and also thought about what has been written on your blog - especially in the light of Luke 7:20-3, and I certainly will have to rethink it, when commenting on this latter passage. Unfortunately, the earliest testimonies about the beginning of Marcion's Gospel are incoherent. Now what to prefer from the choice that we have? The savest option, and that seems to me to be a hermeneutical rule of interpretations is: What does our oldest sources say? (Oldest being Justin, Irenaeus, Tertullian, if we follow the accepted theories; I, in addition, always also check the Synoptics - as, if my theory proves to be right, these would be our earliest readers, editors and, hence, sources for Marcion. But not to end in a circular argument - I only use this step as a final test, not as part of the working out of the text). Having gone for the oldest source-texts, I then look sceptical at what our sources repeat, but is not mentioned by the oldest sources, but could derive from our (canonical) gospels, as I reckon that - as already in Justin - those texts exert some influence (and even more later on). It looks to me (again, needs detailed further work) as if Tatian has used Marcion's Gospel, but also canoncial gospels.
To come back to your proposal with Betsaida. I fully agree that, given the previously mentioned premises, this is a vital option for the identification of Jesus' location of appearance, especially with your ingenious interpretation as 'House of the Demons'. If this came from the Diatessaron, it would not be late, but early, and could, indeed be by Marcion. I agree, it would fit well the content, of what he writes, and also harmonizes with some other witnesses (Irenaeus included) who believe that according to Marcion, Jesus appeared in Judea. And perhaps we need to leave it with this. I have excluded Galilee, also Nazareth - as this information may have been added into our sources by reliance on the Synoptics, but, more importantly, I have left out of the main text any location, because of the conflicting sources, but I will add Betsaida and your description of its potential meaning into my commentary. Why I am reluctant in putting Betsaida into the main text is solely the thought that, if Marcion did, indeed, have Betsaida in his Gospel, why do we have such conflicting evidence precisely with regards to the location. One counter-argument against this of which I can think of might be against the weight of the canonical writings later. And yet, as you still rightly write, Irenaeus still dared to report that according to Marcion the Lord came down into Judea. Maybe a potential solution is that Irenaeus is right (especially as it goes against the Synoptics), and that the Diatessaron with its Jewish background, also knowing of Luke's suggestion, decided to combine Marcion's Judea and Luke's Galilee and turned it into the city which you have then rightly identified as Jerusalem, the place of the 'temple', built by the demons.