On the surviving thing, its super build dependant.
I have 8IF last build and I was surviving couched lances to the head with a morion on.
Yesterday I 1 hit someone at the start of the round using and arb with no extra ammo, so you probably just shot some tanked up fake peasant from a fair distance away.
On the siege shield thing, I totally agree they should last way longer than they do against bolts/arrows.