This is a fascinating passage, and a difficult one. But it is also one of the best examples of the difference in the depiction of Paul, where the Catholic Paul defers to local authority and pleads for his judgement, while the Marcionite Paul declares the judgement he has carried out full stop period without any deference to others.
I think it says a lot also about the development of the church between the two versions where the decision boundaries are defined by the time the Catholic redactor has made his changes, with local jurisdiction over disciplinary measures, and the Marcionite text where a traveling strong leader over a number of disorganized local churches pops in or by correspondence declares the judgements to be taken. This is one of the strong detectable differences throughout Paul one finds when examining the attested Marcionite Paul and the Catholic Paul we received. It strikes me as far more an indication of a passage of time and growth in the size and complexity of the church and it's organization than theological. It speaks to a significant time gap between the two versions.
This doesn't answer your midrashic question, as in this case at least some form of it, if not much of it was present in the Marcionite text; the attestation of this segment is incomplete, so one is left to their eclectic reading of it to determine the original and the later added bits. Instead I'm going to focus on 5:1 and 5:5 below to show the difference in Paul's character.
First καὶ τοιαύτη πορνεία ἥτις οὐδὲ ἐν τοῖς ἔθνεσιν (verse 5:1) is not commented on by Tertullian or Epiphanius, so we have to examine it within the lens of the attested text of the Marcionites and whether it represents something Catholic. In my view this digression does look like it's Catholic. The phrase "even such fornication not heard among the gentiles" suggests a priority of Jew over gentile which is a theme of the Catholic redactor, as being a gentile is implied as being morally worse. Compare this to Romans 2:14 which sets gentiles up as having a law to themselves and doing what is required by nature. We can immediately see the tension between the concepts, and the standing of gentiles. At a minimum I'd bracket it.
What is attested is ὥστε γυναῖκά τινα τοῦ πατρὸς ἔχειν (AM 5.7.2, Non defendo secundum legem creatoris displicuisse illum qui mulierem patris sui habuit
), suggesting the opening phrase of 5:1 is present.
Now while verse 5:3-5 is quoted in DA 2.5 intact, Clabeaux incorrectly attributes it to the Marcionite text. But it's not, as it is spoken by Adamantius the Catholic champion, and not by Markus the Marcionite, who will contradict his version. Without going into great detail it appears that DA part 2 was built on scriptural sayings from a Marcionite source placed in the mouth of a Marcionite by DA's author, but that he has no primary Marcionite source, so instead has his Catholic champion responds using the received text he knows. Such is the construction of DA.
Instead we have three clear attestations, plus secondary scriptural evidence that the original version for these three verses actually read:
τὸν τοιοῦτον τῷ Σατανᾷ εἰς ὄλεθρον τῆς σαρκός, ἵνα τὸ πνεῦμα σωθῇ
"I delivered up this one to Satan for the destruction of the flesh, that the spirit be saved"
The reading ⌐ παρέδωκα for παραδοῦναι is attested by Tertullian and twice in DA by Markus the Marcionite champion:
AM 5.7.2 Sed cum eum damnat dedendum Satanae, damnatoris dei praeco est. Viderit et quomodo dixerit, In interitum carnis ut spiritus salvus sit in die domini
DA 2.8 παρέδωκα τὸν τοιοῦτον εἰς ὄλεθρον τῆς σαρκός, ἵνα τὸ πνεῦμα σωθῇ
Rufinus: Tradidi eiusmodi homiem Satanae in interitum carnis, ut spiritus saluus fiat.
DA 2.21 παρέδωκα τὸν τοιοῦτον τῷ Σατανᾷ εἰς ὄλεθρον
Rufinus: Diende et apostolus boni dei quomodo tradit satanae homines? Dicit enim: Tradidi eiusmodi hominem Satanae in interitum
Further 1 Timothy 1:20 is a pastiche of this verse *in Marcionite form
* (οὓς παρέδωκα τῷ Σατανᾷ).
The Catholic editor changed this direct action of Paul into one that required a deliberative process carried out locally. And one where Paul is giving up his authority to make the final judgement. This is the deferential and unsure Catholic Paul, not the self assured and unassailable Paul with no equal of the Marcionite text.:
For I am indeed absent in body, but present in spirit, have already judged as though I were present, he who has done so, in the name of our lord Jesus Christ. When you are gathered together with my spirit, with the power of our lord Jesus, you are to deliver up such a one to Satan for destruction of the flesh ...
There are other clues that 5:3-4 is a redaction. Paul is no longer present in body. This implies he is dead, a figure of the past. This deliberative group is meeting in his spirit, and invoking the power of Jesus in Paul's absence, in order to perform the task which Paul simply executes in the Marcionite.
None of this touches on the midrashic rewrite point you make, but it strongly suggests it is pre-Catholic. It is Paul who has changed (died it seems). Interesting passage.
Final note: I think the man having sex with his father's wife, is most likely a situation where the man's father had remarried some younger woman (always younger of course), possibly even the same age or even younger than the man in question. The father dies and to appease both families, and not have a sticky inheritance dispute the son takes his father's widow, whom he is not related too at all, as his wife. The Marcionites frowned down on marriage and prohibited divorce and remarriage. Anyway the point is, it's almost certainly not a case of a guys sleeping with his birth mother.