About shaft rating, you can totally fire a rating 5 arrow with a rating 8 bow without destroying said arrow, rules are written in such manners that you just won't be doing a full draw, and be dealing the damages as if you had a rating 5 bow.
Now, for the anti-vampire stuff, I think a rating 8 arrow could still be made of composite materials, with wood included in said materials. Since it wouldn't be pure wood, the allergy would be treated as one rank under the normal one, with a minimum of mild. Not the best thing, but still has its uses. Won't be legit for rating 9 or 10 arrows though, except if they are made in some magical wood that I don't know about but probably exists somewhere.
About the spirit thing, if a spirit possessing an arrow/object is treated the same way as a spirit in physical form, don't they have hardened armor ? Unless you are using low level spirits with a very high strength character, the chance that you are going to deal him damages is unlikely at best, unless i'm missing something.