] Though they all are rotten to each other, it seems they are also willing to work together to get Isabella a good husband.

Convinced as I am that Isabella did not care for Catherine at all (I can imagine what IT said to Miss Andrews about that insipid Miss Moreland), I'm sure she brought it up to John often enough that she was friends with Catherine in order to promote their marriage.
Considering all the schemeing that the Thorpes do, I'll wager that none of them come to a good end. I wouldn't be suprised if Mrs. Thorpe had been just as scheming in her youth and look where it got her!