Except qar'a' is a recognized feature of the language, while qarbe''a' is not. I don't think Klingons who hear qar'a' are thinking that it means is it correct? It would come across to them more like amiright? It's a thing you say, not a sentence to be parsed.

While I agree that Klingon’s likely don’t parse it that way intuitively, the idiomatic understanding is clearly derived from it’s literal meaning.

qarbe’’a’ is both grammatical and parseable — if not commonly heard as an idiomatic expression. Likely {muj’a’} would be more direct; but playing on {qar’a’}, the {qarbe’’a’} seems like it would signal the asker’s increased doubt rather than uncertainty — shades of meaning.