Tense Confusion – Understanding Had + Forget

The problem with this question isn't that it has not received enough attention. It's that it still hasn't been edited to provide the requested context. There's nothing inherently correct/incorrect about OP's cited...

"You had mentioned that your sister had been kidnapped by him."

As pointed out (to the same OP) on a previous question, don't use Past Perfect unless you really have to is a good principle. It might be a slightly contrived context, but we can easily imagine the above sentence being uttered as a "question" (a statement made in the expectation of it being confirmed).

Further suppose the speaker is a detective asking the brother about an earlier conversation he had with someone else. I'm no Agatha Christie, but obviously there could be contexts where the detective needed to know whether that other person was aware of the kidnapping before that conversation took place (or at least, before the point in the conversation that the recounting to the detective has reached).

If the detective had used Simple Past, the brother might interpret the question differently, and answer "Yes" when what he actually meant was he'd mentioned the kidnapping later, not earlier. The entire denouement of a crime story could thus hinge on an incorrect answer to a misunderstood question.

But for most conversational purposes, OP should simply assume that Past Perfect is overused by some non-native speakers, and he should strive to avoid being in that number.

The use of "have" is incorrect here. The use of "had" can be grammatical, but it doesn't quite fit the context. Instead use: "Before that, I completed....". When listing your accomplishments, you want to be concise and use the most active voice possible.