The following are instances where the linguistic parallels between Marcion's Gospel and Mark's Gospel are arguably stronger than the parallels between Marcion and Luke. Although these examples are few, they challenge the traditional scholarly concensus that Marcion's Gospel is merely a redaction of Luke's. They suggest that Marcion and Mark share some source material independently of Luke. 

The Greek texts used in the analysis are Dieter T. Roth's reconstructions in The Text of Marcion's Gospel, and the Society of Biblical Literature's Greek New Testament. For the analysis, I stripped Roth's Greek text of references to Luke, and assigned sequential verse numbers to the positive Greek reconstructions that remained (numbering them in the order that they originally appear in his book). The resulting file is: "04 DT Roth Marcion Greek Reconstructions Only without References." 

"Because They Had Known Him to Be the Christ"

Marcion V12

ἐξήρχοντο [δὲ καί likely present] δαιμόνια ... κραυγάζοντα ... σὺ εἶ ὁ υἱὸς τοῦ θεοῦ. [καί likely present] ἐπιτιμῶν (οὐκ εἴα αὐτὰ λαλεῖν) ...
Were going out [and/also] demons ... shouting ... You are the Son of God. And rebuking (he was not allowing them to speak) ...

Mark 1:34

... καὶ δαιμόνια πολλὰ ἐξέβαλεν, καὶ οὐκ ἤφιεν λαλεῖν τὰ δαιμόνια, ὅτι ᾔδεισαν αὐτόν.
... and demons many he was casting out, and not he was allowing to speak the demons, because they had known him.

Luke 4:41

ἐξήρχετο δὲ καὶ δαιμόνια ἀπὸ πολλῶν κραυγάζοντα καὶ λέγοντα ὅτι Σὺ εἶ ὁ υἱὸς τοῦ θεοῦ. καὶ ἐπιτιμῶν οὐκ εἴα αὐτὰ λαλεῖν, ὅτι ᾔδεισαν τὸν χριστὸν αὐτὸν εἶναι.
Were going out and/also demons from many, shouting and saying that You are the Son of God. And rebuking not he was allowing them to speak, because they had known the Christ him to be.

Shared Omission

Both Marcion and Mark lack the final explanatory clause ὅτι ᾔδεισαν τὸν χριστὸν αὐτὸν εἶναι ("because they knew him to be the Christ") which is present in Luke. This is a significant shared omission against Luke.

Context

The narrative context in Marcion (ἐξήρχοντο... δαιμόνια) is a general statement about exorcisms, more akin to Mark's summary (δαιμόνια πολλὰ ἐξέβαλεν) than to Luke's specific, singular event frame.

This is the strongest case for a Marcion-Mark link, based on a shared structural omission.

"Who Is This Who Speaks Blasphemies?"

Marcion V25

... τίς [δύναται likely present] {ἀφεῖναι ἁμαρτίας} εἰ μὴ μόνος ὁ θεός.
... Who [is able] {to forgive sins} if not only the God?

Mark 2:7

Τίς δύναται ἀφιέναι ἁμαρτίας εἰ μὴ εἷς ὁ θεός;
Who is able to forgive sins if not one the God?

Luke 5:21

Τίς ἐστιν οὗτος ὃς λαλεῖ βλασφημίας; τίς δύναται ἀφεῖναι ἁμαρτίας εἰ μὴ μόνος ὁ θεός;
Who is this who speaks blasphemies? Who is able to forgive sins if not only the God?

Structural Simplicity

The Marcion fragment presents a single, direct question. Mark also presents a single question. Luke, however, features a double question, prefacing it with Τίς ἐστιν οὗτος ὃς λαλεῖ βλασφημίας; ("Who is this who speaks blasphemies?").

The core of Marcion's question (τίς... ἀφεῖναι ἁμαρτίας) is structurally closer to Mark's standalone question than to the second part of Luke's doublet.

The simpler, more direct form in Marcion aligns better with Mark's structure.

"The Son of Man Is Lord of the Sabbath"

Marcion V39

... κύριός ἐστιν ὁ υἱὸς τοῦ ἀνθρώπου και τοῦ σαββάτου.
... Lord is the Son of the Man and of the Sabbath.

Mark 2:28

ὥστε κύριός ἐστιν ὁ υἱὸς τοῦ ἀνθρώπου καὶ τοῦ σαββάτου.
So then Lord is the Son of the Man and of the Sabbath.

Luke 6:5

Κύριός ἐστιν τοῦ σαββάτου ὁ υἱὸς τοῦ ἀνθρώπου.
Lord is of the Sabbath the Son of the Man.

Word Order

The word order in Marcion and Mark is identical: κύριός ἐστιν ὁ υἱὸς τοῦ ἀνθρώπου καὶ τοῦ σαββάτου. Luke inverts the final elements: Κύριός ἐστιν τοῦ σαββάτου ὁ υἱὸς τοῦ ἀνθρώπου.

This is a clear case of identical syntactic structure between Marcion and Mark against a different structure in Luke.

"The Christ of God"

Marcion V136

{ἀποκριθεὶς δὲ Πέτρος} εἶπε· σὺ εἶ ὁ Χριστός [a reference to Christ being "of God" may have been missing]
{Answering yet Peter} said: You are the Christ [...]

Mark 8:29

ἀποκριθεὶς ὁ Πέτρος λέγει αὐτῷ· Σὺ εἶ ὁ χριστός.
Answering the Peter says to him: You are the Christ.

Luke 9:20

Πέτρος δὲ ἀποκριθεὶς εἶπεν· Τὸν χριστὸν τοῦ θεοῦ.
Peter yet answering said: The Christ of God.

Verbatim Agreement

The core confession in Marcion (σὺ εἶ ὁ Χριστός) is verbatim with Mark.

Divergence from Luke

Luke's version is different: Τὸν χριστὸν τοῦ θεοῦ ("The Christ of God"). The Marcion note speculates the longer Lukan phrase may have been absent, which, if true, makes its text identical to Mark's.

This is a very strong verbal parallel where Marcion and Mark share an exact phrase against a different one in Luke.

"That to No One Should They Speak about Him"

Marcion V137

... παρήγγειλεν μηδενὶ λέγειν τοῦτο.
... (he) commanded to no one to speak this.

Mark 8:30

καὶ ἐπετίμησεν αὐτοῖς ἵνα μηδενὶ λέγωσιν περὶ αὐτοῦ.
And he rebuked them that to no one should (they) speak about him.

Luke 9:21

Ὁ δὲ ἐπιτιμήσας αὐτοῖς παρήγγειλεν μηδενὶ λέγειν τοῦτο,
He yet having rebuked them (he) commanded to no one to speak this,

Vocabulary

Marcion and Luke share the verb παρήγγειλεν (he commanded), which is absent from Mark's ἐπετίμησεν (he rebuked).

Atypical Markan Syntax

However, the gross syntactic structure of the prohibition is closer between Marcion and Mark. Furthermore, Marcion uses the infinitive λέγειν (to speak) after μηδενὶ, a construction typical of Markan Greek. But then in this particular verse Mark uses what is for him atypical syntax:  ἵνα μηδενὶ λέγωσιν περὶ αὐτοῦ (that to no one should they speak about him). To complicate the analysis even more, we note that Luke also uses the infinitive here in line with Marcion, which tends to make this a very weak example. However, the μηδενὶ λέγειν cluster is a direct parallel in all three. So, there is a significant linguistic connection. And the example shows that even where vocabulary aligns with Luke, the grammatical construction can still be typically Markan in its directness and simplicity. 

A Long and Convoluted Set of Editorial Processes

This points to a potential stronger Marcion-Mark parallel in an earlier edition of Mark before the cross polination of the ἐπετίμησεν/ἐπιτιμήσας (rebuked) language between Mark and Luke, which might then explain the atypically complicated μηδενὶ λέγωσιν περὶ αὐτοῦ language of this verse as it appears in canonical Mark. Our linguistic analysis must take into account a long and convoluted set of editorial processes leading to the final forms of the canonical gospels. 

Conclusion: Points of Closer Verbal Parallel between Marcion and Mark

Based on a strictly technical and linguistic analysis of the provided texts, the most promising instances for a stronger Marcion-Mark parallel are:

• V12: Shared omission of the final explanatory clause found in Luke.

• V25: Structural simplicity (a single question) against Luke's double question.

• V39: Identical word order in the "Lord of the Sabbath" declaration.

• V136: Verbatim agreement in the core confession ("You are the Christ") against Luke's different formulation.
  
  

These examples demonstrate that there are specific points where the linguistic evidence—in the form of shared omissions, structural simplicity, identical word order, and exact phrasing—supports a closer verbal parallel between the reconstructed text of Marcion and the Gospel of Mark than with the Gospel of Luke. The existence of such examples substantiates—without definitively proving—a theory that somewhere in the editorial processes, Mark and Marcion had sourcing in common that was not transmitted through Luke, which then provides evidence for the thesis that Marcion's Gospel is not entirely—if at all—a redaction of Luke's Gospel.