You are wrong Giuseppe.
We have already listed all the places in the text where the son of man is attested as present in Marcion. But the evidence is even greater than that. For example in Dialogue Adamantius 1.7 (Pretty's English translation) we are given a clear indication of how the Marcionites read the text
Adamantius: Well then, Christ says, "I am the Son of Man" Is He in your view, Son of Man, and not Son of God?"
Megethius: Christ is the Son of God.
Eutropius: You said that what is written must be taken as it stands. How is it, then, that He is Son of God when he calls himself Son of Man?
Megethius: He spoke figuratively (παραβολῇ) when He called himself Son of Man.
Eutropius: Is figuratively to be understood in a spiritual sense, or in its natural meaning?
Megethius: In a spiritual sense.
...
Megethius: Some passages are to be taken in a spiritual sense, and some in the literal.
To be honest, this is hardly different from any other Christian sects at the time, or even today, including the most orthodox. Even modern self proclaimed "Fundamentalists" (e.g., inerrant scripture advocates) are as fickle as the Marcionites, picking and choosing what to read literally and what to read "spiritually".
This brings us back to my fundamental point about the Gospels and their use for evangelism. A sect would use a Gospel that was sufficiently close to it's views that minor theological problems could be explained away by exegesis or "spiritual" reading. However if problems with that Gospels text were directly contradictory to the sects message they were forced to write another Gospel to use. This competition between sects and the need for a sufficiently compatible Gospel to their sect's theology and preaching is why we have four today. There were many more sects than four, but the old adage, "horseshoes and hand grenades" or the American "good enough for government work" applies. To write a new Gospel required the problems with the existing one(s) be too great to overcome with fast talking and maybe a small "textual variant" here and there -- what Bart Erhman aptly calls "orthodox corruptions of scripture".
I have come to a conclusion, not that far from John Calbeaux, that the Marcionites are not the first authors of the texts, but rather an early state of the texts. Where I do differ is that I believe the Marcionite texts were the first "published" editions of the texts of Paul and the Gospels (an early form of Luke) and so have a Marcionite editorial layer on top of them. But not a complete rewrite of any (exception perhaps with Galatians being original to Marcion).
There was a Gospel prototype(s), and it developed prior to and independent of evangelism for some other purpose (IMO as an internally performed play) within the confines of proto-Christian (monastic) communities. The Marcionite author/editor contribution was to take this prototype and turn it into a vehicle of evangelism. But within it was contained many instances of passages referring to the Son of Man. Apparently it was not enough of a problem to excise them, or much of anything else (I suspect perhaps ten verses at the beginning with John the Baptist got axed, as they were contradictory; too many hints of the "last prophet" theology remain in the Marcionite text, or rather the polemic against that theology, to believe it was not known to the author). Instead a layer of Marcionite material (e.g., Lazarus and the Rich Man story, also many sayings incorrectly attributed to "Qeulle") was included.
Megethius provides us with the method Marcionites used to explain away the contradictions. The method worked sufficiently well. The mistake you and many others make is assuming the Marcionites were the originators of the complete text, rather than a sect which stood somewhere in the progression of the text, albeit the earliest published versions of the text (by this I mean publicly known beyond the confines of the pre-evangelism monastic Christian communities). The Marcionites were using mostly already written material well known in Christian circles (small as they were) simply adding a layer with their bits to it. And frankly that is pretty much how all the religious books were written. From scratch writings are rare. Instead it's much easier to come up with an interpretive model. Megethius states how the Marcionites did this.
“’That was excellently observed’, say I, when I read a passage in an author, where his opinion agrees with mine. When we differ, there I pronounce him to be mistaken.” - Jonathan Swift
