You are so right. The irony of John Dashwood thinking that Edward is "unfeeling" to keep to his word and marry Lucy, reveals how unfeeling John Dashwood is.

I love Mrs. Jennings response and also, like Reeba, get a kick out of Marianne's trying to keep her composure.

Another bit that reveals John Dashwood's true (lack of)character:
His own two thousand pounds she protested should be his all; she would never see him again; and so far would she be from affording him the smallest assistance, that if he were to enter into any profession with a view of better support, she would do all in her power to prevent his advancing in it."

Here Marianne, in an ecstacy of indignation, clapped her hands together, and cried, "Gracious God! can this be possible!"

"Well may you wonder, Marianne," replied her brother, "at the obstinacy which could resist such arguments as these. Your exclamation is very natural."