I originally started to write a brief response and then found more and more aspects of the question that could be discussed at this point, even if there may be further discussion later. If that is still a problem, you can remove this post.

That is an interesting perpective. I do agree that from that point of view, Darcy was not totally "in Lizzy's power" -- but he was in Lizzy's power to the extent that she (unconsciously, of course) caused him to do something that was "against [his] will, [his] reason, and even against [his] character." That's not total power, but it's still a lot of power.

I think it is an important point, though, that the issue of whether Darcy was in her power was totally irrelevant to Lizzy's refusal. If Darcy had articulated a humble proposal and even assured her that he was so besotted with her that her relatives would always be welcome in his home, she still would have refused him for the reasons she explains (but that she wouldn't have explained in response to a humble proposal).

However, from the point of view of judging Charlotte's perceptiveness, I think what is important is how Charlotte would have defined the concept, and I don't think that Charlotte would be quite so philosophical about it -- she didn't, after all, say anything about "totally."

I suspect, from what we know about Charlotte, what she considered important about marriage was that it was the pleasantest preservative from want, and it provided a woman with an establishment of her own. What mattered to Charlotte was Lizzy getting that ring on her finger and so from Charlotte's point of view, if Lizzy could get Darcy to propose, he would be "in her power" by Charlotte's own definition.