What would be the best description of the relationship between Walter and his wife?
I think they fit the cliche, "agree to disagree" to a tee. Each have determined their own ways to deal with their irritations of one another and have agreed to stay together. I think all marriages have to be this way to a degree, but theirs certainly does not appear positive.
Although it's pretty sad, at least they have a commitment they are keeping in spite of struggle.
They are like water and oil, they don't mix well, but if you throw a lot of other ingredients into the mix, you just might be able to get water and oil working for the benefit of the consumer. This works in cars (as long as the water's in the radiator and oil is going through the engine) and salad dressings (throw in some sugar and vinegar and the water and oil will play nicely when stirred).
Certainly, an apt description could be a lack of emotional affect. The relationship is featured as one where there is little emotional connection between both and little emotional inquiry as to why the way things are between both of them. It seems that the relationship is one rooted in routine and habit than anything else emotional or substantial. It is this lack of emotional affect that ends up pushing and inspiring Walter to daydream, in that there is little in the real world that can help allow him to find some level of roots or deep bond. The propensity to dream could be a direct response to the lack of emotional affect and connection to his marriage, amongst other elements.
I think that the best description would depend on whether you are Walter or if you are Mrs. Mitty.
I think that Walter would say that he is henpecked. I think he would say that his wife is a total nag who just will not leave him alone.
By contrast, I think that Mrs. Mitty would say that she is long suffering. She would say that she has to take care of herself and also watch out for this loser husband who is always screwing up.
So I guess that their relationship is pretty sad -- they each think there is something pretty badly wrong with the other.