1. Justin was the original author of the material in Against Marcion Books Four and Five; Tertullian received a corrupted text of that treatise edited by Irenaeus
The first principal of my reconstruction of the Marcionite canon is that a corrupt Justinian text is our principal window into the sect. I think Andrew Criddle will follow me up to this radical supposition. To this end, Justin - a user of a 'harmony' gospel - like Ephrem (another 'harmony' gospel user) recognized the Marcionite text was 'like' his own. Thus 'not like Luke' but like the (lost) apostle gospel text.
2. This 'harmony' gospel did not follow the order of the synoptic gospels.
While there is no explicit reference to this fact, the Epistle of the Apostles used a harmony gospel which had a different ordering (see intro of that text). There can't have been dozens of different gospels in the early second century. The gospel of the author of the Epistle of the Apostles was likely related to Justin and Marcion (as Vinzent has already suggested). When Irenaeus references a heretical gospel whose passages are 'scrambled' into a completely different order - it was this same text (Adv Haer 1:8 http://www.newadvent.org/fathers/0103108.htm).
3. In the same way the gospel was all scrambled; the Pauline epistles were scrambled - meaning material from the original apostolic letters was scattered across the various epistles out of their original order specifically to obscure the original sense of the author.
Look at 2 Corinthians chapter 3. There is no logical continuity between 2 Corinthians 1 - 6 and 7 - 12:
In Justin's original treatise (Tertullian Adv Marc 5.11) it is readily apparent that the original letter immediately shifted to Ephesians 2:12:Therefore, since through God’s mercy we have this ministry, we do not lose heart. 2 Rather, we have renounced secret and shameful ways; we do not use deception, nor do we distort the word of God. On the contrary, by setting forth the truth plainly we commend ourselves to everyone’s conscience in the sight of God. 3 And even if our gospel is veiled, it is veiled to those who are perishing. 4 The god of this age has blinded the minds of unbelievers, so that they cannot see the light of the gospel that displays the glory of Christ, who is the image of God. 5 For what we preach is not ourselves, but Jesus Christ as Lord, and ourselves as your servants for Jesus’ sake. 6 For God, who said, “Let light shine out of darkness,” made his light shine in our hearts to give us the light of the knowledge of God’s glory displayed in the face of Christ.
7 But we have this treasure in jars of clay to show that this all-surpassing power is from God and not from us. 8 We are hard pressed on every side, but not crushed; perplexed, but not in despair; 9 persecuted, but not abandoned; struck down, but not destroyed. 10 We always carry around in our body the death of Jesus, so that the life of Jesus may also be revealed in our body. 11 For we who are alive are always being given over to death for Jesus’ sake, so that his life may also be revealed in our mortal body. 12 So then, death is at work in us, but life is at work in you.
Of course it is Tertullian or Irenaeus (not Justin) who now claims that the Gentiles are meant. When we reconstruct the original narrative this is clearly not so. Paul is addressing a community who think they are Israel - perhaps 'the new Israel.' In the context of 2 Corinthians 3's reference to the Israelites and what happened on Sinai we read:And so, even though it were, The god of this world, yet it is of the unbelievers of this world that he blinds the heart, because they have not of their own selves recognized his Christ, whom they ought to have known of from the scriptures ... He (Marcion) in fact has not observed that the conclusion of the sentence is in opposition to him: Because God, who commanded the light to shine out of darkness, hath shined in our hearts, unto the light of the knowledge of himself in the person of Christ who was it that said, Let there be light? And of the giving of light to the world, who was it said to Christ, I have set thee for a light of the gentiles,p those in fact who sit in darkness and in the shadow of death?q To this, by foreknowledge of the future, the Spirit answers in the psalm, There hath been set as a sign above us the light of thy countenance, O Lord. Now the countenance of God is Christ the Lord: and of him the apostle has already said, Who is the image of God. So then if Christ is the countenance of the Creator who says Let there be light, then Christ and the apostles and the gospel and the veil and Moses, and the whole sequence, does on the evidence of the end of the sentence belong to the Creator, the God of this world, and certainly not to him who has never said, Let there be light.
I forbear to treat here of another epistle to which we give the title To the Ephesians, but the heretics To the Laodiceans. For he says that the gentiles remember that at that time when they were without Christ, aliens from Israel, without the association and the covenants and the hope of the promise, they were even without God, were in the world,s even though <they were> of the Creator. So then as he has said the gentiles are without God, and the god they have is the devil, not the Creator, it is clear that the lord of this age must be understood to be he whom the gentiles have accepted instead of God, not the Creator of whom they know nothing.
This is only one example of how Adv Marc can be used to reconstruct a completely different ordering of the Pauline epistles. But most importantly it identifies Irenaeus as misrepresenting or perhaps filtering what the Marcionites believed through his own theological concerns - i.e. a profound monarchian interest.And even if our gospel is veiled, it is veiled to those who are perishing. 4 The god of this age has blinded the minds of unbelievers, so that they cannot see the light of the gospel that displays the glory of Christ, who is the image of God. 5 For what we preach is not ourselves, but Jesus Christ as Lord, and ourselves as your servants for Jesus’ sake. 6 For God, who said, “Let light shine out of darkness,” made his light shine in our hearts to give us the light of the knowledge of God’s glory displayed in the person of Christ Who said Let there be light?
At that time you were separate from Christ, excluded from citizenship in Israel and foreigners to the covenants of the promise, without hope and without God in the world. But now in Christ Jesus you who once were far away have been brought near by the blood of Christ.
14 For he himself is our peace, who has made the two groups one and has destroyed the barrier, the dividing wall of hostility, 15 by setting aside in his flesh the law with its commands and regulations. His purpose was to create in himself one new humanity out of the two, thus making peace, 16 and in one body to reconcile both of them to God through the cross, by which he put to death their hostility. 17 He came and preached peace to you who were far away and peace to those who were near. 18 For through him we both have access to the Father by one Spirit.
19 Consequently, you are no longer foreigners and strangers, but fellow citizens with God’s people and also members of his household, 20 built on the foundation of the apostles and prophets, with Christ Jesus himself as the chief cornerstone. 21 In him the whole building is joined together and rises to become a holy temple in the Lord. 22 And in him you too are being built together to become a dwelling in which God lives by his Spirit.
So the Marcionites clearly identified a 'god of this world' and a god of heaven in exactly the manner of the ancient Israelites - it was Irenaeus who found this offensive and did his best to obscure their beliefs.