Didn't read the thread but I don't actually care! Jumpshot is nothing OP. Skilled archer has two possible ways to win a noob shielder. 1) jumpshotting 2) sidestepping. Ofc veteran shielder will not let archers to wiggle past his shield anyhow because he knows this and keeps his shield always in a right position.
This really depends on the relative agility of both players.
A typical 1h guy (no leokinngrim-like) holding his shield up turns veeery slowly (it's fast in a RL context, but not at all in warband) when already moving forward. Archers, with light armor and 8 ath or more, can sidestep and turn much faster.
Avoiding being hit by robinhood, blackbow, etc like that isn't trivial. It takes more guessing and anticipation than reflexes, really. If you did this using reflexes, you would be too late.