@fumanchu I don't know...ἐν τῷ εἶναι αὐτὸν προσευχόμενον ("as he was praying") is clearly singular, and κατὰ μόνας ("alone") appears to be idiomatic, a set adverbial phrase. Technically μόνας is not only plural but feminine, probably due to an understood χώρας or ὁδούς. — Susan ♦ 17 hours ago

If (in the framework of your question) there was no "long ending" to Mark, then its absence cannot have any significance for the development of Matthew and Luke. Obviously (given the scenario you sketch), Mt and Lk were not dependent on Mk at this point. Is there anything more to be said? — Davïd 41 mins ago