Concerning the panda translation, at the point where Law and Sengoku are having a conversation, I read it again and it seems that it is actually correct BUT still sucks. It says "Don't go writing it off as an act of love or something" so it's 2 negatives (Don't+write off)=1 positive. It's just like he is saying to Traffy "It's not like he didn't love you". It is a very complex translation and you can very well misunderstand, unless you read it 5-10 times over :P At least that's how many times it took me, because I was acting like I have so much time to spare, trying to make excuses for the translators :blink::getlost: (i'm having a hard time to believe that people can translate that badly :P )
Writing off isn't a negative. It is a colloquialism that means to discount, disregard, dismiss, or otherwise reject, with an implication of labeling (writing) something as something else.
You can say "Don't write it off," and that is not a double negative - it is the same as saying: Don't disregard it.
Then we have the slightly more complicated, but more common "Don't write it off as…" This means: Don't disregard it, thinking that such and such.
Because of this complicated independent/dependent clause structure (don't disregard subject1, thinking that such and such), we can't just translate "Don't disregard" as "Do regard;" or, at least it isn't helpful to. See below)
So an example: "Don't write me off as too young."
This definitely doesn't mean I am too young (on your analysis, it does). it means don't disregard me, thinking I'm too young.
(Double negative translation would be: Do regard me, thinking I'm too young. The thinking I'm too young no longer does any work - we just have a command to regard me, and follow it with something the person thinks, but we don't connect the two - we don't say that we should or shouldn't regard them for that reason or that we are right or wrong to think I'm too young.)
In One Piece, the example is: "Don't go writing it off as an act of love or something." Well, we need to know what "it" is. "It" was Coras saving Law (generally) as well as his self-sacrifice (specifically).
He's saying not to disregard or dismiss what Cora did, thinking it was love.
There is a connection here between the command and the "thinking…" that gets lost when we translate it as a double negative. That results in "Do regard what he did, thinking it was love," and tells us nothing about whether or not he is right to think it is love.
It's a weird thing to say. Sengoku definitely wanted to make a claim about whether or not Cora loved Law, and on the double negation account, he doesn't - He just says, yeah you think he loved you, maybe you're right I have nothing to say about that, but regard what he did. And that just isn't how "Dont write it off as..." works. That just shows how badly Panda screwed the pooch :P