Does my acceptance to another university—after applying to it while I am enrolled in my current university—count as taking the opportunity of another student and an injustice to her, given that I wish to withdraw from my current university immediately upon my acceptance to the other university?

If you did not violate university regulations and the mediation did not cause a deserving person to be deprived, then there is no harm in that, and it is not considered taking someone else's right. However, if the mediation leads to depriving those who are deserving, then it is not permissible, due to the injustice and infringement on the rights of others involved.