The Normal/Killing damage mechanics do complicate the game (I'm leaving KA's out of my in-process Champions Lite rules), but they also provide a mechanical distinction between lethal (guns, knives, etc) and less-lethal (fists, batons, etc) damage.
I agree that Extra Limb is probably the best way of handling this if there is a mechanical benefit. Otherwise it's just a Distinctive Feature (or not, if that's not unusual for your campaign).
If I was becoming a Themed Avenger, I would choose one of the military-derived arts (Commando training, krav maga, etc). I'd be less concerned about the philosophical aspects and more concerned with how to take a bad guy down.
When creating my Impact! setting, I debated having one of the cometary fragments hit near Las Vegas. I decide that would kill too many people, and dropped it north and east of Reno instead.
?
One of the neat things about Hero is that you build the power you want, rather than picking from a list of existing powers. You have to approach this from a different angle.
So, leaving the game mechanics aside for the moment, how do you want this power to work?
I'll do a mix. Sometimes I'll just file the serial numbers off of a published villain, sometimes I'll use them as-is, sometimes I'll creates one from scratch.