I agree that, “Am I to understand that you hadn’t been following the handball tournament?” is perfectly grammatical English. It’s the use of the past perfect continuous tense in relation to an event that would have happened after you’d been watching the handball tournament.
Your other alternative sentences don’t convey the same time relationship.
However, some ESL students overdo the past perfect and past perfect continuous once they’ve learned them, and they use them in situations where they should use the simple past or the present perfect. Not being privy to the whole conversation, I can’t judge whether this student was using the tense appropriately. By itself, though, the sentence is grammatical.