1. I am not sure about anything. I think it is highly likely. Also supporting this is the reference to 'Antitheses' throughout Tertullian Against Marcion Books 3, 4, 5 with the added dementia of citing Matthew 5:18 at least 5 teams against Marcion repeatedly saying this was removed from Marcion's gospel. The implicit understanding is that the rest of Matthew 5 was in Marcion's gospel but not removed.
If the six "you have heard it said" statements in the Sermon on the Mount are part of the Antitheses, then Marcion did not create them from scratch.
Clearly Tertullian's many statements make clear the Marcionite contention was that Paul wrote the gospel which contained these statements not Marcion. The bit about Marcion is a later orthodox invention/distraction.
The payloads of the final two statements, Matthew 5.38-42 and Matthew 5.43-48, for example, are contained in a different format in Didache 1.3b-5a, and I have a hard time believing that Marcion has anything to do with that.
You believe in many Judeo-Christian traditions/myths. Everyone is entitled to their presuppositions. I am only advocating we see the Marcionite understanding clearly and fairly and not according to orthodox BS. Again, the Marcionite understanding is that Paul wrote the gospel that had the antitheses. Then the BS from Galatians 1, 2 is read by Tertullian (= Irenaeus) to say that Paul wrote a gospel which agreed with a written gospel held by the Jerusalem church. They approved of his WRITTEN gospel and sent him on his way. With respect to your Didache business the original orthodoxy of Irenaeus's reading of Galatians 1, 2 would imply the existence of two parallel gospels with presumably two antitheses. Just stating the original or close to original orthodox understanding - at least before the denial that Paul wrote a gospel started.
Maybe I am wrong, but I would need something to go on. If this observation holds true, then Marcion constructed his Antitheses out of existing materials.
See above. Tertullian (Irenaeus) makes clear TWO gospels written independently of one another one which was at Jerusalem among the pillars and another written by Paul which he humbled himself for an hour to allow them to read and approve.