Perhaps it was because Estelle's offensive capability was revealed, the process of getting Minerva and Heine to be accepted as her mentors were far easier than what they first thought.
Not only the Clareste couple, other residents of the manor also deemed that her current teachers would only hold her development back. Estelle was clearly far more adept than the tutors.
Alan only administered some limitations and regulations for the teaching methods, most of it concerning Estelle's safety. He had already acknowledged the fact that his daughter was superb in what she does, and did not intend to impose on that matter.
To tell the truth, the five years prior to the invasion was enough for Estelle to memorize the magical spells that were exposed to the public. She even had the opportunity to learn unique magics thanks to Minerva and Heine.
There was one glaring fact that bothered her every time she cast a spell.
The magic spells that existed were simply put, impractical.
