I really wanted to ask, why did the emperor (Li Rong’s father) give her the illusion of choice in whom she wanted…
it's because her father is threatened by the power of the noble families hold. Li Rong came from a powerful noble family which is the Shangguan family. If she got married into a noble family, the emperor is afraid that the noble families would be unstoppable/uncontrollable in the future. And here comes Pei WenXuan who came from a not so prestigious family. He became a perfect consort to balance the noble families