Since it was created by Dr. Okrand and he is terrestrial, I don't think we can call it "truly extraterrestrial". But I suppose it is at least possible that he created that bit of grammar out of need rather than copying another language. Since it's impossible (or at least extremely unlikely) for one person to know all the grammar of all the languages, I suppose we may never know for sure.
Yes, maybe not extraterrestrial, but possibly unique. Dr Okrand has pointed out that all of these features are found in human languages, though not in this combination. I know of lots of languages that mark the topic, Japanese and Korean being the most obvious examples, but every language that does puts the topic at the beginning of the sentence. Klingon is quite interesting in putting it at the end. Cicero often creates suspense in a speech by making very long sentences that finally end in the principal verb (as Latin often does), wrapping up the entire thought. A Klingon orator could produce a similar effect, giving the hearer all the characteristics of a topic, but only clarifying at the end of the sentence what the topic is. I know it does not sound like a very Klingon thing to do (more Romulan, maybe), but it is an interesting possibility of the language.