lol i dunno. i think they have their OH MY GOD THEY SHOULD GET MARRIED BECAUSE... reasons and they have some OH MY GOD NO THEY SHOULD NOT GET MARRIED BECAUSE... reasons. honestly i have no idea whatsoever even after reading all the books. i think they should because i think they love each other. im pretty sure cinder loves kai but im not sure if he loves her. i think he does though but there was nat a lot of details about him. i think they shouldnt because for one, all the people on earth (and luna, if cinder becomes queen) will be all like OH MY GOD NO WE CAN NOT HAVE OUR EMPEROR MARRY A CYBORG LUNAR FUGITIVE!! most of the problem there is that most of the people on earth and luna think cinder should die and that she shouldnt marry kai because she is a lunar cyborg fugitive. people might even be angry because cinder would not be able to have any kids, as mentioned in either scarlet or cress. this would prevent her to have any more heirs to the throne. sure they could adopt, but people might get angry because that unlucky kid would not have any royal blood or family.
You make a good point about the political problems that would come into this but really they are just PERFECT. Both characters are amazing-- Cinder is a mechanic cyborg Lunar princess revolutionary, and Kai is Emperor of the Eastern Commonwealth. There could be some awesome assassination plots from the cyborg-hating minorities, and the Lunar prejudice, but those people overlook the fact that Cinder SAVED THE WORLD and then ABDICATED the throne of Luna. And of course she would survive them because she's Cinder-- in fact, it could be great fanfiction material. About the relationship being one-sided-- there is so much both characters could build off from each other. Kai is a bit derpy in the series, but what I love about him is his capacity for leadership while Cinder was still unseure about people looking to her. There's much she could learn from Kai's attitude towards his responsibility to his people and his country-- he's the most patriotic person in the series. Likewise, Kai could learn from Cinder's drive and the suffering she went through as a ward of Adri-- he's made his fascination with her quite clear. Kaider FTW.