Sombre, as for executing prisoners, It is not my analysis, it's what these researchers came to, and they can base their theory either on some historical evidence, or not. But It is hard for me to imagine they would invent such idea like reason of execution, without any evidences. Though, I don't know the reality. French and English historians can see reasoning of execution quite differently. It's quite normal in history. We know the "truth" mostly from those nations who could write better or who won
As for training of longbowmen...I don't really think it takes years to train, at least if we speak about English longbowmen. Otherwise they wouldn't cost twice less then swordsmen (it's historical fact from records of vassals , who supplied their people to English army for this battle.)
There is a big difference between training a sportsmen, who needs to shoot with high level of precision, and squad of archers, whose task is much simpler - just position the bow in a proper angle to horizon and use certain amount of force to deliver an arrow on a distance which he is commanded to shout. Though it requires force, it is not real "targeting" there. You are just taking part in doing shower from arrows, not shooting into the apple on a head of a princess.