Well, so I read it.

I don't think Colonel Brandon picked up any sign of Elinor's carefully guarded emotions. Also, Elinor esteems the Colonel for this offer. She appears glad for Edward although she wished she did not have to be the one to bestow it.

Yet the narrator tells us, 'her esteem for the general benevolence, her gratitude for the particular friendship, which together prompted Colonel Brandon to this act, were strongly felt and warmly expressed. She thanked him with all her heart'.

My impression was Mrs Jennings made a fuss of Elinor being engaged to Col. Brandon. Curiosity sharpens her hearing and from snatches of converse, she jumps to the assumption Col. Brandon is making Elinor an offer of marriage !

Mrs Jennings thinks Col. Brandon not only meant to do poor
Mr Ferras a kindness, but is willing to wait 2-3 months for Edward to officate at his wedding after Edward's ordainined.

'Lord bless you my dear!-Sure you do not mean to persuade me the Colonel only marries you for the sake of giving ten guineas to Mr Ferras!' (ch.40)

After this, Elinor immediately explained 'by which both gained considerable amusement'.
When Elinor mentions Col. Brandon doesn't think the living will allow Edward to marry, Mrs Jennings makes her remark,
'The Colonel is a ninney'. ;)
She feels Edward and Lucy can marry on a small income.