You have really one down the rabbit hole, then from that created a false set of choices. Brilliant piece of "misinformation." Applause from the peanut gallery for that. Now to unravel and set things correct. First off, why are you going back to the fallacy of Western text = Marcionite text? This is demonstrably false. Secondly, why are you assuming if there is a Marcionite text it must be original (Klinghardt's folly)? When you start with false conditions you can only reach a false conclusion.
Let's start first with the problem of the Western text. Bezae Codex Cantabrigiensis is a bit of a wild text. While yes it does have some early readings, especially of note the Western Non-Interpolations, it also has many late variants and dozens of expansions, many from the margins. This is after all a 5th or 6th century manuscript, which dates 300 to 400 years after the Marcionite gospel's writing.
Now I am going to suggest a far more probable explanation for the appearance of the insertion of this variant in Luke 4:31 that accepts the Marcionite text as reported by Tertullian, Irenaeus, Adamantius and Epiphanius. Let's assume the Marcionite text read as they say:
]Ἐν ἔτει δὲ πεντεκαιδεκάτῳ τῆς ἡγεμονίας Τιβερίου Καίσαρος, [ἡγεμονεύοντος ποντίου Πιλάτου τῆς Ἰουδαίας,] κατῆλθεν εἰς καφαρναοὺμ πόλιν τῆς γαλιλαίας.
In the fifteenth year of the reign of Tiberius Caesar [, Pontius Pilate being governor of Judea] he descended into Capernaum, a city of Galilee.
note: only one witness has Pontius Pilate included in the Marcionite opening. It's plausible it was included, as the Marcionite text places Jesus' origin story in terms of Roman authorities, not Jewish. Luke seems to draw the Jewish officials from the Ebionite gospel text which uses them to place Jesus, but doesn't mention Pilate.
In the fifteenth year of the reign of Tiberius Caesar [, Pontius Pilate being governor of Judea] he descended into Capernaum, a city of Galilee.
note: only one witness has Pontius Pilate included in the Marcionite opening. It's plausible it was included, as the Marcionite text places Jesus' origin story in terms of Roman authorities, not Jewish. Luke seems to draw the Jewish officials from the Ebionite gospel text which uses them to place Jesus, but doesn't mention Pilate.
Now let's ask why an editor might want to add "near the sea, in the region of Zebulun and Naphtali" (τὴν παραθαλασσίαν ἐν ὁρίοις Ζαβουλὼν καὶ Νεφθαλίμ)?
First off, the words are lifted verbatim from Matthew 4:13, which places it after mentioning that Jesus left Nazereth and came to dwell in Capernaum. The association is unique to Matthew, and it was made for a very specific reason. Matthew wants to tie Jesus to the already known Hebrew God of the OT, and to show how Jesus was prophesized ahead of time, not coming unannounced. Matthew shows why he made that association in verses 4:14-16 where he quotes Isaiah 9:1-2
to fulfill what was spoken through Isaiah the prophet: "The land of Zeb'ulun and the land of Naph'tali, on the road (ὁδὸν "the way/path") to the sea, across the Jordan, Galilee of the Gentiles-- the people who sat in darkness have seen a great light, and for those who sat in the region and shadow of death light has dawned."
In Matthew the association is rather striking, in that it harkens back to the kingdom of David's time. But then again Matthew is strewn with deliberate associations to OT passages in order to use quotes of it as proof texts. By comparison the Marcionite text, as we know it, is devoid of such proof texts, and the gospel uses modern (at the time) Roman markers to create the timeframe of events.
If we apply the principle of Occam's razor, it would seem far more likely that the editor of Codex Cantabrigiensis harmonized the Lukan text to Matthew, than for it to have been standing in the Marcionite text. One can in fact speculate that the association started in a proto-orthodox scribe's footnote placed in the pre-Lukan version of the gospel in Marcionite form, because Jesus appears from nowhere into Capernaum, rather than coming from Nazareth.
Bottom line, this gloss is not Marcionite. Further, you have fallen back into the folly of associating a particular text type with the Marcionite text, when in fact no such association exists (per Clabeaux). This is simply a gloss in a wild text, where a scribe took a marginal note and brought it into the text to harmonize with other gospels, a frequent occurrence in Codex Cantabrigiensis.