] Okay, so here's where my new idea comes in. What if James only thinks it is mutual consent? From the earliest moments of the aquaintance between the Morland siblings and the Thorpes, JA establishes a pattern in which the the Thorpes say things they really don't mean and the Morlands, taking them at face value, misunderstand them completely.

I’m trying to catch up here with all the posts from last week, but this thread caught my attention. I went back to James’ letter and I think I have found another proof that Isabella wasn’t absolutely sure about FT.
Here’s a quote from the letter:
“Her duplicity hurts me more than all; till the very last, if I reasoned with her, she declared herself as much attached to me as ever, and laughed at my fears”
She wouldn’t have to deny her infatuation with the captain if she was sure, would she? She should be glad to get rid of James ones she had used him to attach F. According to James’s letter she was still playing both cards, and as Henry predicted on chapter 18, she eventually messed up.
Henry:
“It is probable that she will neither love so well, nor flirt so well, as she might do either singly”.