Okay, I really hope I’m not ruining your post further, and I don’t claim that this is a particularly original thought, but hear me out:
In ANH, Leia has a British accent when she’s still trying to bluff her way through Vader and Tarkin by pulling senatorial rank, and drops that accent when she becomes so agitated that the bluff falls apart, and she doesn’t pick it back up again.
I’d argue that this implies that Coruscant’s social elite tend to speak with a crisp British accent, and Leia adopts it when acting as a senator, because when in Rome and beat them at their own game and all that, but it’s not her normal accent.
And since Mara Jade is my fandom speciality, I can’t help but bring her into this by pointing out that she grew up within that Coruscanti elite - the Emperor’s own court, the most elite of elite, even - which would imply that her natural accent is British.
Now, she certainly had speech training that would allow her to speak naturally in any number of accents, because she often acted as an undercover agent where her own natural accent would be a dead giveaway, and she left Coruscant not long after the battle of Endor and then, in her own words, spent nearly five years “sloshing around the rotten underfringes of the galaxy”, surely places where she would want to conceal the upper crust accent that could potentially put a target on her back. Arguably she would continue to do so even after she joined Karrde; she would often be working with people and in places where such a social marker could be to her detriment. So probably by the time of HoT, Mara is so accustomed to speaking without it that her natural accent has effectively been replaced (presumably with the general American accent of the Big Three, which seems to be the accent equivalent of Basic within the GFFA).
But when Mara is really agitated, I’ve always imagined her slipping back into the crisp British accent she was presumably raised with, much the same way that, when she was really angry with her father, Hera Syndulla slipped back into a Ryloth/Twi’leki accent.
That’s my headcanon, anyway.