When the shield is on your back, it's also Body armor + 10 to projectiles aimed at the shield, so you could go in heavy armor with a shield on your back and just backpedal to archers and you'll take no damage, unless they shoot you in the legs or anywhere that the shield doesn't cover.
Not correct it adds the [shield armour+ 10] to your body armour if you are hit in the shield by a projectile.
Example. You got 50 body armour and a shield with 15 shield armour on your back. An arrow hit it and your total armour is 50+15+10=75 armour.
Axes do double damage to shields. Don't remember if this is before or after the the bonuses from PS and STR, but I believe it to be only the raw damage of the weapon (so even better).
About armour calculation and damage types. I'm actually unsure about this. The obvious answer is that it's the same as for normal armour (blunt/piercce bypassing more armour) but that really ought to be the other way around for shields. Could a dev or someone who knows answer this? It would be nice to have another damage reduction calculation for shields were cut is the most effective, then blunt and then pierce.