I think we both agree that "logia" (as per Papias) means "sayings and doings" and not just sayings, and though you may not agree that that there actually was an original "Hebrew Matthew" of which "various Greek texts claimed to be translations," thus far I think this could still be the case. It just looks like the Ebionites' version was "quite divergent" due to their vegetarian doctrines and such (though perhaps the same could be said, with respect to orthodox doctrines, about the NT version).I think that there was an early tradition that someone named Matthew penned the Logia, and that ... Various Greek texts claimed to be (translations of) Matthew's Logia, or at least to bear some formative connection to Matthew.
But at the same time I'm fascinated by how similar (if not entirely) the Ebionites' version is to Mark regarding Jesus' baptism. It makes me appreciate what you mean about discerning what is "Markan" in Matthew or vice-versa. Now I'm starting to think that maybe Mark and Matthew are more interrelated than I had been thinking, in the sense that maybe they could both be passing on (independently and in their respective ways) the same "logia." But at the same time as that, it still "seems" (to my amateur eyes) that at least the NT Matthew does (also?) incorporate and "correct" Mark, beyond just independently sharing the same "logia" as Mark.