@Godly: No, sorry mang.
1) Like has been pointed out above 0 dmg =/= no damage. Also, you convert damage into wounds. You then prevent the wounds with a prevention flip (see pgs 44, 48). The damage has already been done. The ability clearly states "when damaging" not "after wounding." Could this be a simple lexical error on Wyrd's part? Most definitely, but as currently stated the decapitate effect happens after damaging.
2) Decapitate is a kill effect so no, you wouldn't get a second Slow to Die action, even if you had healed with your first. Kill effects don't reduce you to 0 wounds and then remove you, they just remove you regardless of Wounds.