Let's talk about how Edgeworth addresses Maya, because I think it's super interesting!
"Ms. Maya Fey" when introducing her to the court. He's a professional, after all.
"Maya Fey" on a separate occasion.
Does he ever call her "Ms. Fey"?
No. He does address Mia as "Ms. Fey!"
As a matter of fact, Edgeworth uses "Ms. Fey" to only address Mia. He never addresses Maya as "Ms. Fey".
What about after this trial?
He just calls her Maya!
And to her face? Still "Maya".
And then in 3-5, still "Maya Fey"/"Maya".
Besides reserving "Ms. Fey" for just Mia, I think there's a really interesting reason he calls her "Maya" so informally considering his formal tendencies.
Yes, it involves the Japanese version of the game.
"Mayoi-kun...."
"A. Mitsurugi kenji." / "Oh. Prosecutor Mitsurugi."
In Japanese, Edgeworth calls her "Mayoi-kun" after 1-2, where he addresses her as "Ayasato Mayoi" during her trial. If you have a basic understanding of Japanese honorifics, you might be familiar that "-kun" is usually reserved for men and boys. Maya (and Mia) call Phoenix "Naruhodo-kun", as an example. They both use the honorific to imply that Phoenix is a subordinate (which is a touch scandalous coming from Maya, considering she's both younger than him and not even a practicing attorney, but Phoenix never corrects her.) It can also be used for male classmates in school.
There is a reason, however, that women are sometimes addressed as "-kun", and that's in the workplace! It's common for male coworkers to call their female subordinates "-kun", especially if they are a more familiar coworker.
(Edit, slight correction/addition: addressing a female coworker as "-kun" is more common and even seen as more polite than "-san" for a subordinate in settings like parliament and courts!)
As for Edgeworth, he certainly caught that Phoenix addresses Maya as "Mayoi-chan", but it's probably too familiar for him, so he wants to use a more respectful honorific for her, and something more familiar than "-san", which is similar to "Mr."/"Ms." So, he calls her "Mayoi-kun"! (He also calls Kay Faraday "Mikumo-kun", for similar reasons.)
So I can see why the localizers would scratch their heads as to how they would localize "Mayoi-kun". The nuances of using that honorific can't be translated easily, so between calling Maya "Ms. Fey" (and probably also agreeing that ought to be reserved for Mia) and just "Maya", they ultimately decided to just stick with her first name.
