I don't have to kill her, but I really want her robe. I haven't used companions before. Would their kills count for me? Would they be any more effective than a Vampire Matriarch and Patriarch? I'm not sure I know of any poisons or spells that could kill without me getting credit. Also they'd have to do massive damage since I haven't seen her health bar move even when she had the Matriarch and Patriarch wailing away on her.
Read my edit above.
To answer your questions:
Companion kills don't count for you.
Companion effectiveness varies on their stats, equipment, and if they're invincible/essential or not. Most of the time companions end up being nothing more than meatheaded meat shields that rush head long into battle.
Poisons don't give you credit for the kill.I used to love hitting guards with powerful poisons and killing a town's entire guard and only getting like 40-200 bounty.
Scripted spells and items... I can think of a few examples.
Midas Magic has a spell which summons rocks to crush enemies.... not sure if credit is given to you or not.
Lost Spires has a bear trap that takes half of a NPC's health away with each use.... the player doesn't get credit for the kill
Thievery options:
You could always use the NPC's yield mod and use something like companion share and recruit to have her give you her robe, or whatever.
There should be a mod that allows you to completely loot unconscious enemies.
You could sneak up on her and reverse pick pocket some 0 weight black robe on her and then leave and come back later hoping she wears that instead of her earlier robe, and then you can pickpocket the robe you want off of her.