James and Isabella led the way; and so well satisfied was the latter with her lot, so contentedly was she endeavouring to ensure a pleasant walk to him who brought the double recommendation of being her brother’s friend, and her friend’s brother, so pure and uncoquettish were her feelings, that, though they overtook and passed the two offending young men in Milsom Street, she was so far from seeking to attract their notice, that she looked back at them only three times. (Ch. 7)

Good point about needless theater being the beginning and end Isabella’s character. I agree with you that Isabella’s veracity is questionable. The narrator ironically describes Isabella as uncoquetish highlighting that she is a coquette as defined in Johnson’s 1824 dictionary, page 152 : A gay, airy girl; a woman who endeavours to attract notice. Isabella hints she has a preference for a clergyman, wants to see no one but him and is betraying her feelings by speaking too much of him but it is all very artful and belied by her coquettish behavior. I think Isabella’s bright and “incessantly challenging” eyes show she wishes to attract James’ notice but her divided attention between him and the offending young men suggest that in Ch. 7 (at least) she has little personal regard for him. If that continues to be true I hope James will be more on his guard because a lack of affection does preclude a lack of ambition.