Both Addy and Chelsea have extremely good reasons not to kill witches; Addy can copy their power, and Chelsea can convince them to use their power for her own purposes.

Yes, the slaying should be limited to witches you are particularly vulnerable to. You can't get much more vulnerable than being exposed to people who can instantly make huge changes to your value system against your will.

I suspect this would be enough to lock them in a stalemate.

Not if they are sane. You don't give people free reign to hack with your values.