I guess I always assumed she knew about his proposal to Anne, but considering how tight-lipped the family is, it's possible she didn't...on the other hand, his family may have known, and in a small community, everyone knows everything about everyone else. Mary only never found out about CW because she was at school at the time.

But I think that Mary often deludes herself--I'm thinking of a chapter-jumping thing, right now, when she convinces herself that she is an excellent mother, while doing quite the opposite...I don't want to reveal anything here, but I think Mary could have easily convinced herself that she would have been Charles' first choice, but he initially thought her too young, or was afraid she would never accept him, or...she could convince herself of any sort of nonsense that showed herself in a good light--and all this without the cooperation of Charles, who would never want her to fill her head with delusions!