Am I right in interpreting the part in bold?

In she had closed her eyes in selfish relief, in selfish relief is describing the manner (how?) she closed her eyes. "in selfish relief" could also be expressed by "with selfish relief" which may be more comprehensible for non-natives, but less idiomatic. The author means that she was relieved by the fact that her husband was invalided from the army, and this was selfish because, although her husband must have gone through physical suffering and lost his career, she was only thinking of her own advantage of having him back home.

As for the rest of the passage in bold, you can see a ellipsis there:

As long as they lived, she vowed to herself, she would never again be parted from him, her resolve (being) an expression of her gratitude for this kind misfortune.

Her resolve not to depart from her husband for the rest of her life is the expression of her gratitude for the fact that he is coming back home due to unfortunate circumstances. I like the use of the oxymoron kind misfortune. It is a misfortune for her husband and for her (if she shares in the suffering of her husband in any way), but it is a kind one for her, because they are reunited.