Regarding the attempt to illustrate scribal tendencies by examining Matthew's use of Mark: as I said already, this is not a legitimate comparison, because Matthew was an author, consciously using Marcan material as source-material; Matthew was not operating as a copyist. This should be obvious.
cJW: "Where intent is limited to copying, I would agree that omission is more likely than addition."
Great; we agree. (I'd say /generally/ more likely -- the ratios are not overwhelmingly one-sided. And I'd say that one of the very points where scribes got into trouble was when they veered from their strict duty to duplicate the contents of their exemplars. But /generally,/ yes.)
JW: "Regarding "Matthew" tending to shorten "Mark's" stories, I hear that but I don't see it."
That's because you've just looked in Mark chapter 1. And even there, you've overlooked places where Matthew moved Marcan material to other parts of his account (instead of conveniently keeping it all in sequence).
Try the presentation at http://www.textexcavation.com/synopticlistedinventory.html
, which will allow you to see out-of-sequence parallels, as well as verbal similarities in the Greek text -- things that you just can't do at the Five Gospels comparison-site you mentioned. Select 10 or 12 places throughout Matthew where Matthew's account is paralleled in Mark, and see whether it is Matthew or Mark who has more words within that episode.
Here are a few episodes to consider:
The Healing of a Leper: http://www.textexcavation.com/synhealleper.html
The Healing of a Paralytic: http://www.textexcavation.com/synhealparalytic.html
The Gadarene Demoniac: http://www.textexcavation.com/synexorcismgadarene.html
Of course three other episodes could be cherry-picked that would show the opposite results. But if you go all the way through the parallels between Matthew and Mark, and look at the word-counts (which are listed at the site, episode-by-episode), you will see that although Matthew's episode is sometimes longer (especially when he adds material from another source), usually the longer episode is in Mark. Which implies that when Matthew is using Marcan material, Matthew is usually condensing Marcan material, even though he sometimes supplements the material he has condensed. This should be indisputable, since it is a simple matter of counting words.
JW: "Is this directly relevant to Scribal editing of source texts? Yes it is."
No; it isn't. Writing a book using source-materials is a different task than reproducing an already-written text.
Getting back to Mark 16:9-20 --
JW: "Since "Matthew" is longer than "Mark" anyway you look at it and preferred, the tendency would be to edit "Mark", the shorter text, to "Matthew", the longer text, and therefore addition here would be more likely than subtraction."
And thus the tendency would be not to make just any addition, but an addition that looks like what we see in Matthew: an ending in which the frightened women are met by Jesus, and recover their resolve to report to the disciples, who believe the women and therefore proceed to Galilee. But that's not what we have in Mark 16:9-20, is it. We have, instead, a sudden restart in 16:9, followed by scenes in or near Jerusalem, even though this creative editor/composer you posit (or, rather, Kelhoffer posited) had the means, motive, and opportunity to use Matthew to create a harmonious and smoothly-transitioning conclusion. Do you really not see the difficulty?
Yours in Christ,
James Snapp, Jr.