Relation is feminine, so it should be la relation supérieure-subordonnée idéale.
No, I think not. Consider “la relation père-fils idéale”.
Père and fils are not adjectives that have feminine forms. If you put the sentence into the grammar checker at French spell checker – grammar and spell check - Reverso it will correct it to supérieure-subordonnée.
I would say they are adjectives that are being used as nouns, and as such they are not conjugated by the grammatical gender of ‘relation’.
Automated grammar checkers can only help you so far. This case is very complicated, even for humans.
If you still don’t agree with me, you can always try and discuss the issue with the author of the phrase over at Tatoeba. Please report back here afterwards