Lt. General Hansen wrote:
Currently, in the original, the war with RI was settled right before Nathan's father was murdered. Nathan's father had been installed partway into the war, and had been the one to agree to the peace treaty with RI. At the ball celebrating peace between NB and RI, Nathan's uncle (Edric) set fire to the palace, and used the chaos as cover for the murders. The war with the rest of the US was settled under Edric sometime after, during the 10-year break between the prologue and the main story. It's assumed Edric would not have agreed to the peace treaty, since that's not his style, but it was already signed when he took the throne.
Hm! The impression that I'd gotten (though I've no real idea how ...) was that the war with the rest of the US was settled some time earlier. The basic story that my imagination made to fill in the blanks was that the less- and later-committed colonies agreed to a peace treaty that gave them independence but made significant concessions (a guaranteed protectionist-level tariff on Dutch trade? Paying the taxes on the tea spilled in Boston Harbor?), but the most-libertarian patriots in New England weren't willing to go along with that and kept fighting until it was obvious that it was sign a treaty of some sort or lose the war entirely. (And if Edric was a military leader on the British side, his ruthlessness would be a factor there.)
Lt. General Hansen wrote:
Now, here's something that may or may not help us figure this out. One rough spot in the history of the original book was: Why does everyone hate Edric, but nobody suspects him of murdering Nathan's father?
In other words, nobody likes Edric, especially not on the RI side. They all resent that he took the throne, and he rules rashly and harshly. But for some reason no one--not even anyone from RI--suspects him of having illegitimately stolen the throne. It's a bit of a weak spot.

No one
that we've seen has suspected
aloud that Edric usurped the ducal throne. And since two of the POV characters know the truth, the others are all too young to have been aware of politics at the time of the coup, and situations where this could come up have been rare, that doesn't necessarily limit you.

Your post made me think of several possibilities:
- Some people at the time did think he had them murdered, and he somehow managed to "prove" his "innocence" strongly enough that he's been left alone on the issue since.
- Some people accused him of doing it, and forces and/or courts "not under his control" had them arrested, publicly tried, and executed for slander and lèse-majesté
- People do think he did it ... and thus keep quiet about it, because accusing a tyrant with a larger army than any of his neighbors and possibly even a secret police of a particularly heinous crime he hasn't admitted isn't good for one's health
How
ever, on the list of points on which the expressed positions or lack thereof of the Rhode Islanders strains belief, this is not the one I'd put at the top. Maybe not even in the top five. Probably number two or three on the list of points
just about Edric.
(Not that I have such a list already made up ... but you, O Author, can probably guess what's firmly at the top, given what issues I have belabored at length in comments to you already.

)
Lt. General Hansen wrote:
The story that Edric puts out is that Nathan's parents were suspected of treason, and when confronted about it, resisted. The resulting skirmish resulted in the palace fire and their deaths. In other words, Edric murdered them and claimed it was in self defense/casualty in battle (the area was still somewhat of a war zone, after all). I had also considered having him blame the patriots--the patriots attacked the palace--but I don't think that made it into the final book.
It didn't ... at least not anywhere in the first 195 pages.

Lt. General Hansen wrote:
As for Nathan, I also have to explain why he never tries to contact England and get their help in restoring him to his throne. Part of the explanation is that he's afraid to make any move, because England is a long ways away, and if Edric catches him first, he'll surely be dead. The other explanation is that NB carefully controls all the trade routes back to England, which could make sending mail unsafe--reasonable, but not a complete excuse. The other explanation was that Nathan feared he wouldn't be welcomed in England, because the rumor still exists that his parents were French sympathizers and traitors.
The British didn't control all the trade routes before the war, when they were in nominal control of the entire coast from Hudson Bay to Florida; Dutch-imported tea, for instance, was notoriously-widely available. That George and Nana wouldn't know
whom to contact, or reasonably feared that Edric would have an agent in all the New British, Rhode Island, and nearby harbors and/or every British harbor they would know how to get to London from, may even make a little more sense than why they were staying in New Britain in the first place.

There's also the fact that the British political situation was uncertain at the top (though the most mercurial days of the Regency were, I think, a bit later than your Period); if there was any suggestion that Edric
personally enjoyed Royal favor, seeking Royal justice would have been obviously a fool's errand.
Lt. General Hansen wrote:
(Of course, Nathan doesn't WANT his throne, which is important, but there needs to be a good reason why his caretakers never did it for him.)
I'm wondering if it would make more sense if, instead of Edric murdering them in cold blood and just coming up with a cover story, that Nathan's parents were actually tried and hung for treason. The accusation that they were going to ally with France could still be false, but Edric's tracks would be better covered. Then it would make sense why Mark and a handful of others are the only people who know Edric did everything illegally, and no one else suspects otherwise.
The prologue is IMO one of the strongest concepts in the book, since it introduces two of the main characters (or is it three? I don't remember if Mark's introduced there ...), introduces the setting, shows off Edric's ruthlessness, shows Nathan getting his scar, and demonstrates the fragility of the political situation. A ... better-organized ... coup like you describe here might not fit into that elegant idea so well.
Lt. General Hansen wrote:
Maybe the rumors of French allegiance are even true. Either way, if his parents were actually tried and hung, that would explain why Nathan never reached out to England. The family name has been disgraced.
Perhaps it's merely the pervasive influence of one particular favorite fanfic and the fandom from whence it sprang (and maybe some other fanfics in other fandoms), but I've gotten the impression that in cases of high treason involving nobility, it is at least not guaranteed that the demesne will pass to any of the traitor's heirs, let alone someone as close as a brother or a son. (And given, again, the unpredictability of Royal decisions, I suspect Edric's advisors, if not he himself, wouldn't want to risk a decree that the duchy had reverted to the crown instead of allowing him to inherit it ...)
On the other hand, another idea I had of one possibility of why nobody publicly suggests that Edric had his brother murdred is that he had enough connections in the government in London to have a fraudulent bill of attainder (i.e. printed by the real official printer, with some of the necessary signatures from the real people and the rest forged ... or have Parliament pass a real one, which seems less plausible) issued against his brother.
Lt. General Hansen wrote:
Or, perhaps... In the current story, Edric lied about Nathan's death (even though the lad escaped), just so that there wouldn't be any holes in the story. But maybe Edric didn't lie about it; he made the true report to England that some dissenters had escaped with the son, and so there's a bounty on the head of Nathan's adoptive parents, if not himself as well. That would again explain why Nathan is insistent that his true identity never be discovered, because his adoptive parents would probably face arrest and death.
Since by remaining in New Britain they already face the risk of arrest and death, and that risk comes true in the book, if this weren't something with a volume already in print I would say it was six of one and half-a-dozen of the other, but as it is I recommend leaving this point as is.