Shouldn't it be kwa herini since it's more than one person leaving?
That would be grammatically (more) correct, yes! :)
"Bye", and "Goodbye", are synonyms, extremely common, and interchangeable. "Bye" should be an acceptable response.
(although because of heri meaning "luck/blessedness" I personally would prefer to keep the good in to carry some of the meaning)
Then again, the "good" is always implied in the phrase in English, it adds no extra meaning and thus a language learner won't feel the difference you want it to carry.
Actually some questions before this in this Practice segment accepted "Bye" as a correct one. It should be consistent.