] I think you have hit on exactly the irony of what JA was illustrating. Theory and real life seldom resemble each other.

I think there are greater differences than similarities between the two situations. It's one thing to think it a good thing delay one's departure merely to please a friend. I seriously doubt that, had the subject come up in the earlier conversation, Lizzy would have argued, even in theory, that one should marry -- or not marry -- merely to please a friend.