The fact that we know what the charm does and have SEG would probably work pretty well for defense. I doubt Mo would be dumb enough to use a charm that Zhi made against Zhi himself. That's just begging for him to exploit the weakness in it. Also it would piss him off if she used it against him because he'd know what the charm does and would later recognize it had been used against him.
Mo's philosophy is to engineer situations so that when she stabs someone they have to publicly thank her for not twisting the knife.People have this weird thing about Mo where they assume the worst possible actions from her even when it doesn't make any sense as a thing for her to do, goes against the personal philsophy of conflict she's espoused to us, and is literally to her sole detriment despite all prior characterization of her.
Mo is nice, but I wouldn't call her good. I know that she wouldn't point the charm at us directly for the reasons quoted, but indirectly is another story. She hasn't done near enough to convince me that she's benign or that enabling her to such a degree would really help anyone but her.
Not that it super matters. If we make her a charm it will likely be on commission at a later date and the design will be out of our hands. Mo's next commission will likely be going to whoever wins the tournament so I don't expect anything anytime soon. Plenty of time to see more of her character.