but it's beautiful of list monad ;)

If you consider it acceptable to not keep original order, there'd be no wrong answer.

It is generally taken as a good thing not to swap identical ( up to case-insensitivity ) elements.

Not saying this isn't an issue, but if it hasn't been fixed in four years, it's unlikely to be fixed at any time. Closing.

(>>=) is concatMap. Does it make the code more readable? No.

Sure does! Upvotes on your quality solutions is one of the best ways to earn honor, it's 1 honor for each upvote.

I can't replicate that, but I tried to ensure that the test has as clear of a type as possible. Unfortunately, I believe that's the best I can do.

I had tried to make them slightly more tight, but I just reverted that addition. Try again?

You do not need to be good at haskell to consider this idiomatic.
In my mind this is more about understanding how folds work and not writing their definitions for each individual case.
Similar thing is using map instead of defining your function to iterate over lists in a way that map does it. (I mean writing f = map g instead of f [] = [] and f (x:xs) = g x : f xs)
You get used to it after a while :)