Learn English – What does “had long” mean

The usual wording of this idiom is "the pot calling the kettle black". The idea is that the pot is black itself, so it is not in the position of being able to accuse the kettle of being black. A very similar idiom you will probably be familiar with is "he who lives in a glass house shouldn't throw stones [at others]", or any variations thereof. Wikipedia has more similar idioms still.

The pot-kettle situation is a shorthand to refer to that idiom. It is not a common shorthand, so you won't find it in a dictionary. It is just a hint by this particular author at the actual idiom.

Yes, it both "sounds wrong" to a native speaker, and it doesn't make sense.

The phrase "[person] has been to [place]" simply means the person was present in the place at least once. It contains no information or implications about when they were there, for how long, or why. "has/have been to" is completely without duration. Asking "how long" makes no sense.

The question "Have you been to [place]?" is a question with a "yes" or "no" answer. Are you asking how long ago the person visited? Are you asking how long they stayed in the place? In either case, one would ask those questions instead.