I'm sure she has her reasons. I don't know how strong the Xing kingdom is but she must surely think that if they were to go against Kouka they would lose so she doesn't want her people to suffer and she probably has some conditions to becoming Kouka s vassal without a fight...
Anyway, I really want to know about the shield and the sword. Could it be Hak and Yoon, maybe? Hak would be the sword and Yoon the shield, protecting them by gibing them food and healing them!
To be honest I have a very bad feeling of what would happen to yona after she awakens. I think that so won will be the king and that yona will end up with hak and that peace would be brough. After that ,I have a feeling(sadly) that the four dragons might die (even Zeno) and in the end yona will end up somehow sacrificing her self. I don't want that to happen, but it's just a theory that we should hope never comes to be. ( ̄へ ̄)
Princess Tao, did she know what it means to be a vassal nation? There must be a reason for it. Otherwise, she's just a traitorous princess like what the other people said. To be a vassal nation means your kingdom is a subordinate of the other kingdom. It usually involves tribute and other things. Did something bad happen to Xing? Such as a threat from Sei Kingdom, a great famine, a crisis, or a bad enough disaster for her to want her kingdom to be a vassal kingdom to Kouka? I try to think this positively and wait for the next chapter and the other princess to appear before commenting further. I hope Yona can be careful with the whole thing because unlike other problem she solved, this involve other country fate. I hope her identity as a former princess will remain unknown.
Finally, when will the sword and the shield prophecy progress??? It's been a while since they dug into the King Hiryuu prophecy.