As you said, wounds are assigned one at a time, yes. So if you have a card that does something each time a companion is wounded, you can do it over and over each time you one of those archery wounds is placed on a companion (since they are NOT all assigned simultaniously).
As far as killing off an archer, though...VERY good question. I would think, logically, that any FP archer killed before the Shadow assigns the fellowshp archery total wounds would NOT count towards the fellowship archery total. So if Legolas bit it when you took all the Shadow archery, his arrow would never fly, and the fellowship archery total would be reduced BEFORE the Shadow player(S) has to assign any of it. But I confess I don't know the definitive answer. I could see the argument for the other side too, in that the FP arrows could be considered already "in the air" before the Shadow arrows hit, and thus Legolas' archery would still count.
Hmmm. Anyone know the actual answer? :hey: