Some of them do x% weapon damage e.g. I think water arrow is 40% weapon damage as water damage. I think you can see this somehow, but I'm not sure how. Perhaps it's when you're in a barter screen.
Not sure but I would expect the damage is weapon damage + bow/xbow skill. Those arrows will do more damage with xbows since they have higher base damage.
Dex doesn't have any effect on special arrows other than allowing you to use the bow/xbow. They all have 100% hit chance and that is the only thing dex affects in this case (except ability to use the weapon).
Other than damage and ability to use the weapon in the first place, there is no benefit of dex for special arrows.
HTH
