Had Boney's luck broken in the opposite direction during the previous decade (as it damn near did), there would have been two things going in favor of England: (a) no significant threat to its national interests in Europe (France would be a broken nation, and the concept of the nation-in-arms probably would have died with her), which would let them throw more effort into the mix; and (b) a lot of unemployed soldiers ready for service as mercenaries.
When Wellington's soldiers got hired for duty in the various wars of independence in Central and South America, they didn't do so out of a profound love for liberty--but they sure as hell changed the face of the Americas, for good or for bad.
Again, this also supposes that England would have WANTED America back.
Like I say, the War of 1812 almost turned into a war for national survival. With a lot more British resources (troops, ships, supplies, and money) available for commitment to America, and the concept of the "nation-in-arms" having proven its apparent non-utility, America might have fallen--or at least been severely truncated. (One potential strategy: the British ignore most of New England and the Canadian border, and instead seize the Maine and the Carolina seaboard--specifically to grab the stands of white fir and live oak, and thus cripple American shipbuilding for a couple of generations, until iron hulls begin to appear.)
Another fascinating what-if: Boney gets a sense of when to quit pushing his luck in 1806 or so. Meanwhile, the American War Hawks put together a sensible military program during the 1804-1811 timeframe. With Europe secure under French rule, and America's full military potential in place, particularly at sea, there existed every possibility that America and France might team up to rid the world of "Perfidious Albion" once and for all. So there's a LOT of potential counterfactuals that could have been in play during that time.
BTW, I think your novel's scenario is a little to pedestrian. My guess: China is going to be the first nuclear-armed nation to fight a full-blown civil war--and that might pose a greater threat to American interests than anything China might be interested in doing to us or anybody else.
There are several things that could lead to a civil war in China ... going too far against us may be one of them. Nothing says it can't occur while they are in the midst of an external fight ... particularly once fortunes turn.
Anyhow, the novel is fiction and those who have purchased and completed it are saying good things. I'll be happy if that continues.
Please tell me more about this...if you have the time
Thanks
W1