My objection is the Ukraine analogy is the Second Great War, which happened. Ukraine is not fighting a guerilla war - it is fighting a conventional war of national defense with heavy weaponry.
Don't forget the Argentine and Uruguayan expeditions.
My guess is Britain, in this scenario, tries the planned but OTL aborted Mexican invasion. Or IDK, Sicily?
No reason I can see that the war doesn't peter out, like after Amiens.
Yes, you are correct, they ethnically cleansed the territories and gave them to Poland as part of the destruction of the Prussian state. Sorry for not making that clear.