Well, you were definitely on the right track... I made the strings horizontal instead of vertical which seemed to make the "set angle toward" work better. Then I just placed them at the arrow x and y, and set the angle to the imagepoints on the bow, and set the width to the distance between the points.
capx here: rieperts.com/games/forum/Arch2.capx
As for telling what part of the target you hit, I was going to say make the target up out of 5 different circles (yellow, red, light blue, dark blue and white) that way you can tell be which one the arrow collides with...
But since your target circles were in even increments of 10 pixels, I just compared the y value of the end of the arrow to the y value of the target and calculated what part gets hit. (this is not always very accurate by the looks of it, but it is a start).