I agree with Post 2. But do you have to argue that she is powerless? If so, the only argument I can see you making is that her only way to have power is through her husband. Unlike him, she can't really just go out and do anything -- she has to propel him to action.
I guess you could also argue that she's powerless in the face of fate. She can try to manipulate events but eventually her fate overwhelms her (just as it does her husband) and they are killed.
I think Post 2 has a better argument than what I just said, but if you have to argue that she is powerless, I'd say this is the way to go.
Your question presumes Lady Macbeth is powerless, something which is not particularly true in Shakespeare's Macbeth. It is she who compels her husband to kill Duncan, it is she who keeps the banquet going when her husband seems to be having a fit (and seeing Banquo's ghost); it is she who continues to play hostess in the face of a tragedy (which she helped cause, of course), among other things. It's true that at some point Macbeth seems to keep things from her and she does suffer the effects of guilt for her role in killing Duncan; however, those don't necessarily make her powerless. Of all Shakespeare's women, in fact, Lady Macbeth is among the strongest and most empowered.