Getting married on the spot wasn't simply because Dragon Blood Girl was lovesick and impulsive; there was another reason.
Her rebellious phase.
The reason Hefei's teacher took so much interest in cultivating her was not due to a deep mentor-student relationship. There was only one purpose for training Hefei: to become the Holy Maiden, using the status of a student-turned-Holy Maiden to gain more influence at the Church headquarters.
As a tool, Hefei was thrilled; she found a new home and was done playing along.
That new home was with Austin, who was anything but honest.
The mentor-student relationship then hit rock bottom, until today, twenty years later, the teacher thought that after years of Hefei struggling from the ground up to the position of High Priest, she would surely understand the teacher and then become like her, thus initiating the thaw in their relationship.