what about the emperor and mage? They are not canon? The sisters won't be with them and will just return to Trovancha? Just Regina with Leo? (best pair, no doubt but still...)

of course , our little prince-hostage. The thing is that the emperors of both empires were dumb, so actually the other emperor chose a son to send to other country as a hostage via a lottery, the prince who they sent actually did have some backing. He is taught by super famous martial artist. Then, when the time comes (ML father will be dying), the ML will leave the country and return to his nation to claim the throne. Within the next 3-5-7 years (i don't remember exactly), he will be restoring the power of his empire. Then the FL's father dies, ML seemingly strikes a deal with other neigbouring country to attack the FL's country, but it was actually more of a threat - give me FL or I will attack you with other country.

he won't end up with princess as well (such a shame!!!), He will end up with the boss of the villains (who helped him to kill brother of Nayun)

no, he won't end up with nayun