Elizabeth says:

"To oblige you, I would try to believe almost anything, but no one else could be benefited by such a belief as this; for were I persuaded that Charlotte had any regard for him, I should only think worse of her understanding than I now do of her heart."

My only way to please Jane with this belief is if I thought Charlotte needed a dependent, someone for whom she would always be necessary and someone for whom her talents (intellect, sense, good management, steady temper) would be put to best use. Assuming she wants such a relationship, and knows she does, she could have a regard for Mr. Collins: the regard one has for a child one cares for. It is not what I would want in a husband, but...

Elizabeth continues:
"You shall not defend her, though it is Charlotte Lucas. You shall not, for the sake of one individual, change the meaning of principle and integrity, nor endeavour to persuade yourself or me that selfishness is prudence, and insensibility of danger security for happiness."

Elizabeth thinks that it would be a violation of HER principles and integrity to accept Mr. Collins (and she is right), but I am not convinced it is an overthrow of Charlotte's. Charlotte has every intention of fulfilling her duties, and making Mr. Collins happy (happier than he might deserve, in fact). Charlotte is taking on a hard task in order for a specific gain. She is not so selfish that she does not still scheme for others - for Elizabeth, for her husband, for her family. I believe Charlotte sees the danger, and has chosen her gamble wisely, still knowing it is a gamble. She might lose, but I do not see that Charlotte is willfully blind. I think Charlotte's gambling brings a better hand than Elizabeth would have brought to the table.