the shield 'force field' is supposed to simulate the skill of the shield user intercepting the arrow (the 'force field's size actually depends on the shield skill of the player btw) its an abstraction, isn't perfect but its better than without- trust me, if you remove it shields are useless.
the crossbow jam seems like an old bug that was stomped out in beta, perhaps its somehow come back.
and yes, each class has its own stats which are pretty much as you expect (overall infantry have better melee, nords are slightly hardier, kergaits are less athletic but better on a horse) the difference isn't massive, but its effect is noticeable.
EDIT: to your troubles blocking, do not compare offline and online play- they are practically different games, bots are training dummies compared to real-life players.
I believe the shield-skill's added 'force field' effect does not apply to melee attacks, only for intercepting arrows, so remember that attacks can actually hit past your shield. and also note that if you lower your shield for even a split second, you leave yourself vulnerable, since your shield takes a small moment to get into a blocking position again (bigger shields are slower, but not really noticeably so)