This is the part that bothers me. Elizabeth feels shocked that Charlotte would accept his proposal and that she never really knew her friend, but perhaps she should have known her friend better. Elizabeth assumes her friend thinks and feels as she does, but IMO Charlotte has made it pretty clear that she doesn't and never did.

In Ch. 18, Charlotte showed that she thinks it appropriate to separate your feelings from your behaviour, especially if an eligible man is in your sights: "Charlotte could not help cautioning [Elizabeth] in a whisper not to be a simpleton, and allow her fancy for Wickham to make her appear unpleasant in the eyes of a man of ten times his consequence."

Back in Ch. 6, Charlotte weighed in on how she thought Jane should act towards Mr. Bingley: "In nine cases out of ten a woman had better show more affection than she feels...Jane should therefore make the most of every half-hour in which she can command his attention. When she is secure of him, there will be leisure for falling in love as much as she chuses."

Elizabeth counters that it's all fine if all you're after is a husband or if a woman has designs on someone without any feelings for him. When Charlotte answers that "Happiness in marriage is entirely a matter of chance. ... and it is better to know as little as possible of the defects of the person with whom you are to pass your life," Elizabeth laughs and says
"You make me laugh, Charlotte; but it is not sound. You know it is not sound, and that you would never act in this way yourself."

It's telling that Charlotte never answered that remark. Elizabeth laughed it off, but it's pretty clear that this is exactly how Charlotte planned to act, intended to act and actually did act.