According to the novel, the former king did not have political power. He can protect her because his only status: former king. He's the best, highest, safest, bet she could have. And the current king already has someone he loves so very likely reluctant to took her as royal concubine. The other choice is to become the head of her clan, which currently can't coz shitty father. She will given position from the king later as the head clan and so far her status as former king's fiance protect her. When she become the head clan, she still not marry to the former king btw.