Just a thought, but couldn't you let the history pan out, but enable him to do what he wanted to do, slip away to America? When was the requirement to be US born to run for President come in? Post war of 1812 there would have been a sizeable anti-British sentiment for him to work with. Could you finish with your heroine as First Lady in the Whitehouse with Mr President Bonaparte?

Would have the added advantage of a US angle which might help sales. If you want to have a military aspect, he could always invade Canada!