My tipping point for me was not with the at least somewhat understood interpolations in Josephus, but with
the text of the Christian apologist Justin Martyr. Specifically chapter 26 of the first Apology ascribed to him. This is the famous passage, which says
And, thirdly, because after Christ’s ascension into heaven the devils put forward certain men who said that they themselves were gods; and they were not only not persecuted by you, but even deemed worthy of honors. There was a Samaritan, Simon, a native of the village called Gitto, who in the reign of Claudius Cæsar, and in your royal city of Rome, did mighty acts of magic, by virtue of the art of the devils operating in him. He was considered a god, and as a god was honored by you with a statue, which statue was erected on the river Tiber, between the two bridges, and bore this inscription, in the language of Rome:—“Simoni Deo Sancto.” And almost all the Samaritans, and a few even of other nations, worship him, and acknowledge him as the first god; and a woman, Helena, who went about with him at that time, and had formerly been a prostitute, they say is the first idea generated by him. And a man, Menander, also a Samaritan, of the town Capparetæa, a disciple of Simon, and inspired by devils, we know to have deceived many while he was in Antioch by his magical art. He persuaded those who adhered to him that they should never die, and even now there are some living who hold this opinion of his. And there is Marcion, a man of Pontus, who is even at this day alive, and teaching his disciples to believe in some other god greater than the Creator. And he, by the aid of the devils, has caused many of every nation to speak blasphemies, and to deny that God is the maker of this universe, and to assert that some other being, greater than He, has done greater works. All who take their opinions from these men, are, as we before said, called Christians; just as also those who do not agree with the philosophers in their doctrines, have yet in common with them the name of philosophers given to them. And whether they perpetrate those fabulous and shameful deeds—the upsetting of the lamp, and promiscuous intercourse, and eating human flesh—we know not; but we do know that they are neither persecuted nor put to death by you, at least on account of their opinions. But I have a treatise against all the heresies that have existed already composed, which, if you wish to read it, I will give you. Eusubius (EH 2.13) in the 4th century reports the passage to this point, with slight variations
Τρίτον δὲ ὅτι καὶ μετὰ τὴν ἀνέλευσιν τοῦ Χριστοῦ εἰς οὐρανὸν προεβάλλοντο οἱ δαίμονες ἀνθρώπους τινὰς λέγοντας ἑαυτοὺς εἶναι θεούς, οὓ οὐ μόνον οὐκ ἐδιώχθησαν ὑφ᾿ ὑμῶν, ἀλλὰ καὶ τιμῶν κατηξιώθησαν· Σίμωνα μέν τινα Σαμαρέα τὸν ἀπὸ κώμης λεγομένης Γιττῶν, ὃς ἐπὶ Κλαυδίου Καίσαρος διὰ τῆς τῶν ἐνεργούντων διαμόνων τέχνης δυνάμεις ποιήσας μαγικὰς ἐν τῇ πόλει ὑμῶν βασιλίδι Ῥώμῃ θεὸς ἐνομίσθη καὶ ἀνδριάντι παρ᾿ ὑμῶν ὡς θεὸς τετίμηται, ὃς ἀνδριὰς ἀνεγήγερται ἐν τῷ Τίβερι ποταμᾷ μεταξὺ τῶν δύο γεφυρῶν, ἔχων ἐπιγραφὴν ῥωμαϊκὴν ταύτην· ΣΙMΩΝΙ ΔΕΩ ΣΑΓΚΤΩ. Καὶσχεδὸν πάντες μὲν Σαμαρεῖς, ὀλίγοι δὲ καὶ ἐν ἄλλοις ἔθνεσιν, ὡς τὸν πρῶτον θεὸν ἐκεῖνον ὁμολογοῦντες, ἐκεῖνον καὶ προσκυνοῦσι· καὶ Ἑλένην τινά, τὴν συμπερινοστήσασαν αὐτῷ κατ᾿ ἐκεῖνο τοῦ καιροῦ, πρότερον ἐπὶ τέγους σταθεῖσαν, τὴν ἀπ᾿ αὐτοῦ ἔννοιαν πρώτων γενομένην λέγουσι.  Mένανδρον δέ τινα, καὶ αὐτὸν Σαμαρέα τὸν ἀπὸ κώμης Καππαρεταίας, γενόμενον μαθητὴν τοῦ Σίμωνος, ἐνεργηθέντα καὶ ὑπὸ τῶν δαιμονίων καὶ ἐν Ἀντιοχείᾳ γενόμενον πολλοὺς ἐξαπατῆσαι διὰ μαγικῆς τέχνης οἴδαμεν· ὃς καὶ τοὺς αὐτῷ ἑπομένους ὡς υνδὲ ἀποθνήσκοιεν ἔπεισε, καὶ νῦν εἰσί τινες ἀπ᾿ ἐκείνου τοῦτο ὁμολογοῦντες. Mαρκίωνα δέ τινα Ποντικόν, ὃς καὶ νῦν ἔτι ἐστὶ διδάκων τοὺς πειθομένους, ἄλλον τινὰ νομίζειν μείζονα τοῦ δημιουργοῦ θεόν, ὄς κατὰ πᾶν γένος ἀνθρώπων διὰ τῆς τῶν δαιμόνων συλλήψεως πολλοὺς πεποίηκε βλασφημίας λέγειν καὶ ἀρνεῖσθαι τὸν ποιητὴν τοῦδε τοῦ παντὸς θεόν, ἄλλον δέ τινα, ὡς ὄντα μείζονα, τὰ μείζονα παρὰ τοῦτον ὁμολογεῖν πεποιηκέναι. Πάντες οἱ ἀπὸ τούτων ὁρμώμενοι, ὡς ἔφημεν, Χριστιανοὶ καλοῦνται, ὃν τρόπον καὶ οἱ οὐ κοινωνοῦντες τῶν αὐτῶν δογμάτων τοῖς φιλοσόφοις τὸ ἐπικατηγορούμενον ὄναμα τῆς φιλοσοφίας κοινὸν ἔχουσιν. Εἰ δὲ καὶ τὰ δύσφημα ἐκεῖνα μυθολογούμενα ἔργα πράττουσι, λυχνίας μὲν ἀνατροπὴν καὶ τὰς ἀνεδην μίξεις καὶ ἀνθρωπείων σαρκῶν βοράς, οὐ γινώδκομεν· ἀλλ᾿ ὅτι μὴ διώκονται μηδὲ φονεύονται ὑφ᾿ ὑμῶν κἂν διὰ τὰ δόηματα, ἐπιστάμεθα, Ἔστι δὲ ἡμῖν καὶ σύνταγμα κατὰ πασῶν τῶν γεγενημένων αἱέσεων συντεταγμένον· ᾦ εἰ βούλεσθε ἐντυχεῖν, δώσομεν.
καὶ μετὰ τὴν ἀνάληψιν τοῦ κυρίου εἰς οὐρανὸν προεβάλλοντο οἱ δαίμονες ἀνθρώπους τινὰς λέγοντας ἑαυτοὺς εἶναι θεούς, οἳ οὐ μόνον οὐκ ἐδιώχθησαν ὑφ ̓ ὑμῶν, ἀλλὰ καὶ τιμῶν ἠξιώθησαν· Σίμωνα μέν τινα Σαμαρέα, τὸν ἀπὸ κώμης λεγομένης Γίτθων, ὃς ἐπὶ Κλαυδίου Καίσαρος διὰ τῆς τῶν ἐνεργούντων δαιμόνων τέχνης δυνάμεις μαγικὰς ποιήσας ἐν τῇ πόλει ὑμῶν τῇ βασιλίδι Ῥώμῃ θεὸς ἐνομίσθη καὶ ἀνδριάντι παρ ̓ ὑμῶν ὡς θεὸς τετίμηται ἐν τῷ Τίβερι ποταμῷ μεταξὺ τῶν δύο γεφυρῶν, ἔχων ἐπιγραφὴν Ῥωμαϊκὴν ταύτην· SIMONI DEO SANCTO, ὅπερ ἐστὶν Σίμωνι θεῷ ἁγίῳ. καὶ σχεδὸν μὲν πάντες Σαμαρεῖς, ὀλίγοι δὲ καὶ ἐν ἄλλοις ἔθνεσιν ὡς τὸν πρῶτον θεὸν ἐκεῖνον ὁμολογοῦντες προσκυνοῦσιν. καὶ Ἑλένην τινά, τὴν συμπερινοστήσασαν αὐτῷ κατ ̓ ἐκεῖνο τοῦ καιροῦ, πρότερον ἐπὶ τέγους σταθεῖσαν ἐν Τύρῳ τῆς Φοινίκης, τὴν ἀπ ̓ αὐτοῦ πρώτην ἔννοιαν λέγουσιν
But this passage is not reliable and almost certainly not valid. There are several assumptions in the passage that appear out of place for a 2nd century writer in Rome,
(a) Simon Magus is associated with Gitta (Σίμωνα μέν τινα Σαμαρέα τὸν ἀπὸ κώμης λεγομένης Γιττῶν)
(b) The tribute ΣΙMΩΝΙ ΔΕΩ ΣΑΓΚΤΩ (Eusubius reads the Latin SIMONI DEO SANCTO)(c) Simon is related to Marcion through Menander
(d) The passage association with the prior and following passages
(a) The story of Simon Magus with reference to Gitta is nowhere found before Eusubius (early to mid 4th century) repeats Justin's supposed words (clearly his source), with the pseudo Clementines (Homilies 1.15, 2.22, Recognitions 2.7), which also date from the 4th. This begs the question where was this information hiding for the 175 years between Eusubius' witness and the expansion in the pseudo Clementines and the time of Justin? Why were Tertullian, psuedo-Tertullian, Irenaeus, Hippolytus, and other Patristic heresy lists missing this detail?
It is interesting that this detail seems to be related to the story that Simon parents were named Antonius and Rachel (H 2.17 shown below, R 2.7), which suggests the town name of Gitta is related to the story of his A piece of apocrypha unknown its seems before the pseudo Clementines. This additional information about Simon's parents may well be after Eusubius wrote, although Eusubius' writing may have inspired it.
(H 2.17) Σίμων οὗτος πατρὸς μέν ἐστιν Ἀντωνίου, μητρὸς δὲ Ῥαχήλ, Σαμαρεὺς τὸ ἔθνος, ἀπὸ Γετθῶν κώμης, τῆς πόλεως ἀπεχούσης σχοίνους ἕξ
(b) The tribute ΣΙMΩΝΙ ΔΕΩ ΣΑΓΚΤΩ / SIMONI DEO SANCTO which this passage claims is written on a statue on the Tiber river for a statue dedicated to Simon Magus, and that he is regarded as a God is an impossible statement for somebody who knows Rome intimately as Justin should. The actual alter dedicated to Semo Sancus on the Isle of the Tiber, near the temple of Jupiter Jurarius reads SEMONI SANC DEO (found in 1957). Such a mistake is impossible for Justin, but not it seems for Eusubius almost 200 years later living in the east, when Rome was no longer part of his portion of the empire, and then allowing for a generation or two to lapse after the Tetrarchy was established in 285AD (see map below) that such an error would not be easily detected among the immediate readers, since peoples in the East no longer traveled so often to Rome.
This seems to put a lower bound on the passage of roughly 285AD. Well after Justin, but before Eusubius wrote EH, no later than 323 AD.
(c) Although the direct line of descent from Simon Magus to Marcion through an intermediary of scant information except the name Menander (and sometimes Cerdo) is a myth already present in Irenaeus (AH 1.26) at the end of the second century, making it conceivable that somebody of Justin's era could know that, there is the problem of form. The mention of Menander onward is not attested in Eusubius and may be a further addition. It also follows the form of the writings by Heresiarchs' lists of schisms, which were unknown at the time Justin supposedly wrote this passage. Irenaeus was writing no earlier than the late 170s, but more likely around 190 AD, Tertullian in 205 AD, and maybe 235 AD for Hippolytus. It is a form and style that had not yet been invented.
(d) Finally the relationship of Apology 1.26 to the prior and subsequent chapters is pretty much nonexistent. In fact it seems to have no relationship to the rest of the Apology, as nowhere else is Christian heresy in view or pertinent in anyway. It is perplexing as to why the subject would be broached, being unsolicited, when directing a defense of Christianity to a Pagan Emperor who would know nothing of the internal schisms.But this is external.
Internally 1.25 talks of how Christians used to worship a variety of Gods, for example, Dionysus, Apollo, Proserpine, Venus, Æsculapius, and Achilles, whose various episodes of lust and debauchery are covered in this chapter and shown as abhorrent to Christians. 1.27 then contrasts that unlike the pagan acts of temple prostitution, that Christian children are not exposed to this by their parents, for it is considered something done by wicked men (ἀδικῶμεν ) to raise children for prostitution. Nowhere is there place in this argument for a digression into the pious Marcion, who in no way endorsed such practices, and in fact was more extreme perhaps than the orthodox in his condemnation of these. Certainly Justin knew this.
In conclusion Justin Apology 1.26 is an interpolation from the late 3rd to early 4th century from the east. It has no place in the discussion surrounding in chapters 25 and 27. Simply put, this cannot be used as an early witness to Marcion, as it is built upon myths and styles of a much later era, 150 or so years later.