I think "Good question" should be accepted, as it is a common phrase used in the same way, if I have understood this phrase correctly.
OK, true, makes sense. Added now.
But you can easily say "Dobre pytanie" in Polish as well.
Well, generally "A good question" is not a sentence, unlike the Polish phrase. We accepted "Good question" because it's idiomatic.
Very unlikely, because it assumes there is only one question, which we both know. Generally questions are manifold, making 'a good question' more likely. In actual usage, dobre pytanie is closer to 'good question' and 'a good question' to 'to dobre pytanie', purely on amount of emphasis.
A good question The good question .... Seems a bit hair splitting
Yes. As "to" here is just a dummy pronoun, all of those: [This/That/It] are equally good answers.