I know she's in love with Edward, but neglecting to mention the fact--not once in the nine to ten months she's known him-- that he's been engaged for four years is a pretty serious offense, I think!
Even though she thinks that "He had been blameable, highly blameable, in remaining at Norland after he felt her influence over him to be more than it ought to be," Elinor is still tempted to forgive him when she realizes that, " His affection was all her own. She could not be deceived in that. Her mother, sisters, Fanny, all had been conscious of his regard for her at Norland; it was not an illusion of her own vanity."
I can't weigh those two sides and come out with anything like equal or tipping the balance in Edward's favour. What do others think? In her place, I don't think I could be so forgiving, so soon, even if I did believe that he loved me and not her.
It's not like now, when Edward could just break it off with Lucy and marry Elinor. Unless he can somehow get Lucy to break off the engagement, he's stuck with her, literally, for life. The fact that he was probably going to be miserable with his chosen wife would, for me, not really mitigate the fact of the deception and the hurt he had caused through it.