I feel like how you get access to Mirror Stance could use a bit of a rework, potentially even making it a power or passive ability that you get at the start of the game.
My reasoning is thus: The non-violent option for getting past Sigrune was slated to be, at least per my memory, in the coastlands. But the coastlands are only going to be accessible after The Siege of Khor’minos, which means that a grand majority of the fights that you want to be holding up mirror stance to steal powers in are going to be behind you. So in essence this forces anyone who wants to use the power as intended to restart the game and force their way into getting it early and then build their entire character around using mirror stance to it's maximum potential to grab powers.
Thus, I would propose the following: Mirror Stance becomes a sort of passive ability for the champion that is gained at the start of the game, where as part of general combat they have a chance to steal powers as they get used. You could also add in something for defeating enemies also having a chance (or, in the event of one-off bosses, automatically giving you) their abilities. This would mainly be done to ensure that people have the fairest chance possible to engage with the content without having to metagame, restart, or look up how to use console commands to add the powers they want.
Alternatively you could have the area where you gain the mirror stance power serve as a sort of magical training chamber where you can choose to fight the memory of certain encounters in order to try and gain any powers that you might've missed? The main thing is that I wouldn't want the average player to realize that they've missed a not insignificant side activity and then complain about having to redo the entire game.