In the chapters we've read - has she expressed any doubts about Isabella? John, yes, but I don't remember any about Isabella at all. I don't take her accepting the fault for things like being late as having any doubts about Isabella.

I don't see James as a ninny either though. I think he's a nice guy who made friends with John for the reasons given above and was thrilled to go home with his friend who had sisters (I doubt the 40 odd families had many more available girls than they did boys for Catherine after all!). While there he saw the girl who JA positively says had great personal beauty and implies strongly dressed well and fashionably. She seems outgoing and fun and sweeps people up in her version of the world.