] The example you sited is one, but also the fact that she and the Colonel carried on a lively friendship before (and I am assuming afterwards) they married. Lively friendships take lively people, so obviously Marianne is back to her old self somewhat.

And even her old self was not especially lively, really. Maybe lively is not the right word, but I'm thinking of this conversation between Edward and Elinor back in Ch. 17:

"Nor do I think (gaiety) a part of Marianne's (character)," said Elinor; "I should hardly call her a lively girl; she is very earnest, very eager in all she does -- sometimes talks a great deal and always with animation -- but she is not often really merry."

"I believe you are right," he replied, "and yet I have always set her down as a lively girl."

"I have frequently detected myself in such kind of mistakes," said Elinor, "in a total misapprehension of character in some point or other: fancying people so much more gay or grave, or ingenious or stupid than they really are, and I can hardly tell why, or in what the deception originated. Sometimes one is guided by what they say of themselves, and very frequently by what other people say of them, without giving oneself time to deliberate and judge."

Marianne is present as they say this and makes no attempt to deny or contradict them. She is about passion and strong feelings for the things she values and believes in, but it is not demonstrated in 'liveliness' as such. So, the fact that she then enjoys a lively friendship with her future husband hardly seems like she has lost her enthusiasm for life--rather the reverse!

]And we might add the the Colonel is back to his old self, as well, his self before Eliza broke his heart and the whole business with his brother, and the younger Eliza and Willoughby. Poor man, he did have a lot to deal with, but he got his reward.

Yes, the key word there is that he is restored to animation and cheerfulness. He couldn't be restored to a state in which he had never yet existed! He spoke to Elinor earlier about his 'present forlorn and cheerless gravity', also indicating that that was not his usual personality.

]Is it possible that if his wife were always sensible and and never smiling or passionate, the Colonel would be happy?

More importantly--we know how much he loves her and the lengths he was willing to go to in order to ensure her happiness. Brandon is also 'ever mindful' of other people's feelings and seems especially perceptive about Marianne's. Could he be happy if he did not believe that his wife was happy too? Or to think that he had married a pale shadow of the girl who attracted him at the start of the story?

] her whole heart became, in time, as much devoted to her husband, as it had once been to Willoughby.

This mention of 'in time' is something else that is often treated negatively by the dreaded 'essayists' who criticize S&S. However, I think it can be seen in a positive light. The whole of Marianne's 'relationship' with Willoughby lasted about six to seven weeks. He met them in mid-September and he was gone by early November. Marianne felt instantly connected to him and earlier in the novel scolded Elinor "It is not time or opportunity that is to determine intimacy: -- it is disposition alone. Seven years would be insufficient to make some people acquainted with each other, and seven days are more than enough for others." However, she did NOT know Willoughby as well as she thought she did, and actually didn't really know him at all.

Later after Elinor brings their mother to Cleveland and they are discussing Brandon's chances of securing Marianne's affection. Elinor makes the point that the colonel's worthiness does not rest on one act of kindness to Marianne, but instead that people who have known him very well for very many years all love and trust him implicitly.