You describe Isabella very well. I think she is not to be trusted, and her two-faced, man-chasing ways suggest that her feelings for James fall short of genuine affection.

Isabella's insincerity makes a telling foil for Catherine's straightforward simplicity. Thinking about Catherine's character puts me in mind of Mr Knightley's praise of Harriet as "unpretending" and "artless". These are not vivid or glamorous characteristics, but the contrast with Isabella makes me feel the value of them.