That's your plan, huh? It's like giving them an excuse to send him to some godforsaken place in the farthest reaches of the kingdom. After all, if he's nothing but an ordinary soldier, he has to follow orders. Furthermore, they can't very well give him any special treatment. There's no end of excuses this will allow them to give Yona. If I was Kye-Sook, I'd probably send Hak to some place infested with diseases to have him die of natural causes nobody could help with. On the other hand, what kind of a plan is it anyway from Hak's point of view? For a grunt to reach the higher echelons via promotions would normally take decades.
Something doesn't compute here.