If you want her to have MiA, I recommend you go for it! It's 240xp, iirc, not 300xp. But still. Technically, as has been pointed out above, the background you suggest does not reflect the virtue, but nothing is harmed by saying that your maga has equivalent education, though having equivalent reputation is more difficult to explain. PCs are exceptional, however, so if you have some explanation that satisfies your group, fine.
I also notice that the xp-efficient university academic virtues effectively privilege this kind of education above some of the educational systems of other, non-Christian cultures, which is not reasonable for the period.
Art and Academe has a virtue Magister in Medicina, which is the Doctor virtue for medicine; a woman can earn this degree in one or two places, for realsies. That's not exactly the background you propose, however.
Doctor is the most efficient virtue in the game for explicitly granting xps, though there are tight restrictions on how these are spent. The best such virtue for a maga is Gild Trained, which provides 90xp to be spent on pretty much anything; slightly less efficient than a doctorate but more flexible. Baccalaureate also provides 90xp, but, like MiA, technically requires attending a university. Good Parens provides 60xp for almost anything plus 30 levels of spells; not quite as good as Gild training.
Penultimately, my favorite virtue for granting xps is Strong Faerie Blood. It does not explicitly grant xps, but does let you start a character 15 years older with no penalty; that's 225xp taken before apprenticeship (+5xp because you start with Second Sight 1). These can only be spent on general abilities, unless the character also has some virtue that allows xps to be spent elsewhere, such as Educated, Warrior, Baccalaureate or some Supernatural Ability.
Finally, I would not worry about accounting for age, beyond setting age to what is described in the virtue. Normal character creation handwaves exactly what happened before the start of play, and will rarely result in the same characters that would occur through seasonal advancement, in which virtues like MiA don't make much sense anyway.
But all this is a matter of aesthetics.