Seems the command is purely AI bound and can't be triggered by events alone.
Yes, it can't be used as an event action, only as reaction.
Why CharacterAddSkill does not assign most skills I don't know. Maybe it has the same requirements as using skill has; are you sure you regarded all requirements at testing?
You could try it with Osiris.
The problem you mentioned above (CharacterUseSkill after using a skill): it sounds as if you want to script a CharacterUseSkill command for a player character when he's on turn? As long as you control him (and that's the case in combat on turn) he doesn't perform any scripted reaction.
Maybe it's also necessary to use Osiris for your complex necromantic spell.
