My first thought is that this isn't much of a love triangle if Diana can't even get the name of her crush right.
They hadn't interacted that much; seems obvious to me that Diana is hearing "Natori" as a name more familiar to her. Especially with Japan's l/r phoneme.
My point was more that in comparison, Lapis never seemed to need correcting on what to call her and always calls her Natori. Natori has to correct all kinds of people, so Diana just seems like every other person who doesn't get her name right. And more to the point, she doesn't seem to be putting in any effort to get her name right.
Like, in real life, if someone messes up a name and gets corrected it would be pretty rude to keep saying the wrong name.