] They talked as if it were a foregone conclusion that Catherine would do whatever that group was going to do; that they could sweep in with a plan fully hatched and Catherine would simply go along as a matter of course.

Yes, the three of them make all these plans and Catherine is the last one to be consulted, and even then she's not really consulted, but presented with a fait accompli. Why should Catherine have any objection? What else has she to do? And before she met with Eleanor Tilney, that might have been true, but now there is a rival for her time, attention and affection. Instead of noticing that and making an effort to be considerate of her time, they instead use emotional blackmail and coercion. Not the behavior of a true friend.