I may have spoken too soon, however.
Again, a simpler solution, corresponding to scenario ab in my earlier post, is that they were in fact in the Q source, but just happen to be the sayings included by Marcion/Luke, and ignored by Matthew. There are a couple of problems with this solution, however.
The first problem with this solution is that, as described earlier, Matthew would have had no good reason to leave these sayings out of his gospel. We'll leave aside this problem for the moment.
Because the second second problem with this scenario is that it would seem there are simply no parallels to these sayings found in Matthew.
But that is only true of direct parallels.
Many gospel sayings have indirect parallels--indeed, theories such as the Farrar-Goulder hypothesis assume there are. So we need to check to see if the Thomasine sayings supposedly unique to Mc/GLk have any indirect parallels in GMt. If there are, we may be able to drop the direct connection between GTh and Mc/GLk, leaving the Q tradition and the Markan tradition as the only two with direct connections to GTh.
This would be a scenario related to ab, but even simpler: there would be no Thomasine sayings unique to Mc/GLk at all. Instead, anything Thomasine in Mc/GLk would also be found in GMt, whether in a direct parallel, or an indirect one (and any exception would have an explanation based on Matthew's theological commitments). This would be the simpler scenario, hence would be preferred.
It turns out there are such suggestive, indirect parallels in GMt. What's more, they cluster around one another in the same way they do in Mc/GLk, and they fall in the same order. They are found among the Q-material in the Sermon on the Mount, and are as follows:
Mt 5:4 blessed those who mourn Lk 11:27-28 blessed the barren
5:19 do/keep the law do/keep the word
Mt 5:5 inheritance Lk 12:13-14 share inheritance
5:22 angry with his brother with brother
Mt 6:19 lay up Lk 12:16-21 lay up
Mt 7:19 thrown into the fire Lk 12:49 to cast fire
This sheds new light on the reasons why Matthew would have left these sayings out of his gospel; re-examining them, we can see why he would at least have been uncomfortable with the form in which he found them, and we can also see how he nevertheless tried to use them as best he could.
- Lk 11:27-28 ("blessed the womb")--Matthew could have left this out because he wanted to emphasize Jesus' inheritance of the throne of David--remember that Matthew's is the first gospel with a genealogy, which he uses to establish this inheritance. Matthew would have wanted to avoid any suggestion that Jesus' ancestry (whether adoptive or otherwise) was unimportant. To be sure, he includes the Markan encounter between Jesus and his family (Mt 12:46f./Mk 3:21f./Lk 8:19f.), but that one is somewhat gentler in its substitution of a "family of disciples" in place of a genealogical family. It is not an explicit rejection of Jesus' family, whereas the Lukan version is an explicit rejection--blessed "rather" are those who keep the word of God. Indeed, Matthew is concerned with keeping Torah as much as he can, and mentions the 5th(4th) commandment twice (15:4, 19:19). And Matthew may also have felt uncomfortable with the second half of logion 79, "Blessed are those who never bore" (a reference to the fall of Jerusalem) as again, he wants to emphasize Jesus' genealogy.
He also may have carried over the idea of keeping the "word of God" to Mt 5:19, where his followers are admonished to uphold the law. Indeed, Marcion's version of Lk 11:28 uses POIOUNTES rather than FULASSONTES, and this is closer to the POIHSH of Mt 5:19.
- Lk 12:13-14 ("who made me a judge?")--Matthew may include such "unfair" sayings as Mt 12:12, but the Sermon on the Mount is interested in an egalitarian ethic, and promises reversal of misfortune. Matthew may have left this saying out because he didn't want to portray Jesus as totally unconcerned with distributive justice. (Even Luke has to add a warning against covetousness to whitewash the saying somewhat.)
He also may have carried over the concept of brotherly dispute into Mt 5:22-23 (ADELFW, like ADELFW in Lk 12:13) where he emphasizes reconciliation rather than resignation. Matthew appends this to the non-Thomasine Q saying in Mt 5:24-25, but Luke keeps that saying separate, at Lk 12:57-59.
- Lk 12:16-21 ("that very night")--Matthew would of course agree with the ethic of this parable. But remember that Matthew is rewriting a lot of his material into the single unit of the Sermon on the Mount. That sermon is made up of simple admonitions and examples, but contains no extended stories or parables.
(ETA: And of course Mt 6:19-21 also = Lk 12:33-34...what Matthew has done is combine two originally separate Q sayings into one, but Marcion/Luke has kept them separate, in Lk 12:16-21, 33-34 = GTh 63, 76.)
- Lk 12:49 ("to cast fire")--Matthew's Jesus does not judge while he is on earth; rather, he judges after the Kingdom of Heaven has arrived. "Fire" in Matthew is associated with judgment, and with punishment of the wicked after the coming of the Son of Man. So Matthew would not have wanted Jesus to say he came to cast fire on the earth, for that is reserved for a later time, the time of the Son of Man.
Of course, all of this opens up the possibility that the Thomasine sayings unique to GMt nevertheless have indirect parallels in Mc/GLk. I am not actually aware of any such parallels, but I am willing to consider that there may be some. This would still be compatible with the framework we just established; it would just increase the Thomasine sayings shared by GMt and Mc/GLk, and decrease the ones unique to GMt. It makes little difference for the HSH.
Thus, I suggest (in part with Schuermann) that these sayings were originally found in roughly this sequence in the Q source. Maricon/Luke copied them more or less as they appeared, but Matthew exercised a little more creativity (unusual, but by no means without precendent) as he integrated them into the unified sequence of the Sermon on the Mount.
Next we'll examine these sayings (and Lk 17) from a Marcionite perspective, to make sure that we can find reasons why Marcion would have kept the verbatim versions in his gospel.