I knew that would get you started.
As you remarked perfectly, the early harquebuses used to have rather small bores, ca. 12 mm. As there are so few around it is vey rare indeed to find one of such small caliber.
I also fully agree with you in that the stocks do look rather provincial. I think maybe Romania had the barrels and locks delivered from Nuremberg and other manufacturing centers while the stocks were home made.
As to pressing the lateral push button trigger, this was quite certainly done with the index finger, just as in later trigger development. Using the index finger allows one to keep quite a good grip of the stock with the thumb and the rest of the hand.
I am afraid that my own pondering has not led to any different explanations of the snap lock cocks moving either backwards of forwards into the pan than those considered by you. I think the old gunmakers were just trying, offering both methods to be tested by the shooters.
Thank you again for all these brilliant questions,