I don't know how this difference can be resolved. The difficulty here, though, is that since Mt and Lk certainly chose materials from OrTr that they approved of and that fit their own narrative projects, it can always be argued that saying X, since it fits Mt's or Lk's particular ways of looking at things could have been invented by Mt or Lk for that purpose. The simple observation that a SpLk saying fits into Luke's agendas won't distinguish between those possibilities. I worry about the impact of cleverness on these sorts of things. Human minds find order in chaos by instinct.
Clever minds are particularly good at that.
It worries me about Valentasis' book on Thomas that he can show the tight ideological consistency within Thomas' apparently chaotic sayings by interpreting them all in a certain way. So, as mentioned above, we know that there are Lukan factors at work in either the selection of material from tradition or in the invention of material. Nobody has ever done this.