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).