Returning to the OP’s question about earlier introduction of SMGs in 1914.
Technological change sometimes is imposed from top-down, but more often necessity is the mother of invention.
Top-down introduction of SMGs could have started with Kaiser Wilhelm’s affinity for his hunting carbine based on a Mauser 96 pistol with a long barrel, etc. Whilhelm favoured the Mauser carbine because of his withered left arm (damaged at birth).
This POD starts with Prussian officers demonstrating their loyalty to the Kaiser by privately purchasing Mauser carbines. After a few especially his viscious fights in somebody else's House, they start issuing Mauser carbines to NCOs and specialist troops.
As “gun Jesus” .... er .... Ian McCollum over at
www.forgottenweapkns.com has repeated observed - clamping on a butt stock dramatically improves accuracy!
ATL In a necessity-is-the-mother-of-invention, speculate on some internicide squabble within the Austrian-Hungarian Empire: Sebs versus Croats, Bulgarians versus Lutherans, whatever ..... with Budapest and Vienna severely restricting arms imports to rifles and hand guns. Vienna cheerfully ignores local squabbles as long as they don’t involve artillery or MGs, because it easier to ignore feuding when no buildings fall. “I see nothing.”
After observing the effectiveness of the few Mauser 96 carbines available, some Albanian (?) garage mechanic starts hammering together simplified pistol-caliber carbines. After a few miss-fire on full-automatic, troops deliberately modify triggers to fire full-automatic.
Bonus points if most of the feuding occurs at short ranges within towns.