Elinor started at this declaration, and was sorry for the warmth she had been betrayed into, in speaking of him.

Did Elinor feel like Marianne tricked her? Or did Elinore realize she betrayed herself? I'm inclined to think that Elinore got caught up in defending Edmund's tastes. If she had been in company I don't think this would have happened. Between sisters her guard was down.

To answer some of the questions you pose, I don't think they would suspect Elinore of keeping an engagement secret. She keeps to proper decorum and doesn't show more than is polite. Mrs. D and Marianne are hoping for happiness in a time of sorrow. They see an amiable, eligible bachelor and in their romantic sensibilities blow things out of porportion.
Marianne was astonished to find how much the imagination of her mother and herself had outstripped the truth.