You might have read the whole thread and found out that the editor does not work with the EE and that there is no alternative for it.

You might google for some 'Cheat Engine' scripts for the Enhanced Edition that can be used to modify some things while the game is running.
Since I never used Cheat Engine myself, I cannot give you any info on how to use it and what can be changed with them. You'd have to find out yourselves. Nor can I say if the Death Knight Bane spell can be applied with them.

It's really weird though that the mirror would break before Leandra's spell has been acquired.
If I remember correctly, I left the mines once before getting the spell and could enter there again. But then, my memory might fool me ...