To her credit, Elinor did show some rational optimism back at the end of chapter 16: His coldness and reserve mortified her severely: she was vexed and half angry; but resolving to regulate her behaviour to him by the past rather than the present, she avoided every appearance of resentment or displeasure, and treated him as she thought he ought to be treated from the family connection. I infer that she's anticipating some Edwardy goodness in the future, not just treating his pleasantness at Norland as a fluke.

Incidentally, something like 'sanguine expectation of happiness which is happiness itself' turns up in Northanger Abbey chapter 10, too: The morning had answered all her hopes, and the evening of the following day was now the object of expectation, the future good.

I suspect that Mrs Dashwood and Catherine Morland are both being linked to the saying "It is better to travel in hope than to arrive"**... and that they're both being used to poke fun at its solemnity, Mrs Dashwood with her reckless optimism, and Catherine for her moment of triviality over her gown and head–dress.