PP I think you made a good point generally. On mumsnet, as in any other meeting place, people do bandy around terms such as passive-aggressive without the qualifications to diagnose. As adults we should all understand that whilst acknowledging that they can form a very useful shorthand to convey our thoughts. But, yes to be treated with a dose of salt (mind you I would treat a diagnosis such as that with a pinch of salt whatever the source - defining these are very tricky!).
Detachment, as you have said does have two slightly different usages. As a psychological term it is recommended as a strategy for codependents (amongst others). In more common parlance it means to withdraw, to invest less in a particular thing, perhaps to be more independent.
For cheese I suspect this may be a very useful suggestion. I suspect that she has been in a a codependent relationship however, I do not actually assume that to be a bad thing at all! For many people it works and works well. Nonetheless focusing on herself as she has absolutely no 'control' over her husband can only be good. Cheese you can't make him fall in love with you, but you might be able to be the kind of supportive loving partner that he can fall back in love with - does this make the distinction clear? More to the point you will start to work out what you want and be that person.
I do think that your husband has been 'better' than many we read about here. He is perhaps (who knows, really) trying to explain that something is wrong and he is not sure what to do. I think recognizing that there is a problem and taking steps to address it (seeing a counseller - God I hope he has a good one!) is brilliant. I feel sure it indicates some hope for Cheese's family, whether they stay together or not, to continue to raise their children in a happy environment.
Like WWIFN I suspect that books are not the answer, like AF I think that Cheese needs to bring into her life things which make her happy but do not depend solely on her family.
I hope this kind of summary reflects what you have as individuals said, but also focuses back on anything that might help Cheese (who I suspect is still reading!). Cheese, I have a close friend who has been exactly where you are, her husband was depressed, missing the romance of limerance, and in reevaluating his life (MLC) thought his marriage was not good enough. They are now (probably about 15 years after the original declaration) still together, very happy and making quiet but romantic plans for retirement. If you feel it is worth hanging on in there my friend would say go for it. She did. I would say go for it, but take steps to make sure that you will be happy either way.
Don't give him forever though, at some point you will find it utterly soul destroying living with someone who says they are not sure they want to be with you. make sure you stay alert for that point.
Bloody hell what an essay - Cheese you take care of yourself. You sound bright and determined enough to be able to take what you want/need from these exchanges.