Charlotte may have been simply checklisting the differenced between Col Fitzwilliam and Darcy when she thought of the existence of Pemberley livings. Recall that the association of Charlotte's accompanying Mr. C almost daily to Rosings and the possibility of additional livings is Lizzy's thesis. Lizzy is probably right, but she might be wrong. Charlotte may have gone with her husband primarily to keep him from embarrassing himself even more than usual. She would not want to lose the society at Rosings if it was the only society they had.

Ironically Charlotte has many qualities in common with Lady Catherine; both like to be on top of things, to manage things, to give advice. Both are astute observers, say what you will about the choices they make. Being highly pragmatic, Charlotte IMO stores up as much information as she can about what is going on around her. I can easily see Charlotte keeping all options open.