Imagine it's the final battle of your campaign. You're facing Chejop Kejak in Yu-Shan. His bodyguards and minions, his students and summons and constructs, lie broken and beaten and banished. But the man himself is stronger and more terrible than ever; he's been withering the hell out of you while you've been killing his mooks. The four of your brace yourselves as he prepares to unleash his ultimate technique, and...oh, he pushed the button.
Alright guys, time to narrate the story of how you failed to kill Chejop Kejak. I need a submission from each of you. And of course I, as the Storyteller, get to submit a proposal for my own consideration.
Sucks, eh?
No decent ST would do that. And the strongest condemnation of a Charm I can come up with is "don't worry, it's so unfun that it won't actually get used".
The problems are of a similar kind when a PC uses it, but they're so much worse in the hands of an NPC.
This is almost exactly not how this works. If the ST is the sort to be that badly heavy-handed, they can just give the boss 60 HL and a spammable trans-perfect defense and attack. The only way to make this work as you have laid it out is to have the ST talk out of both sides of their mouth and say that the OSoI stylist can be offered an unconditional victory by the ST and it's not
my fault that the ST also happens to be the stylist's player and also I get to select this unconditional victory.
Assuming that the ST is at least attempting to play in good faith, here is how that would actually play out:
After collecting everyone's thoughts about how the scene could play out, the ST lays them on the table for discussion. Perhaps none of them involve the PCs being injured, if they do it was the ST or a player suggesting it because it makes for a dramatic exchange where the PCs get a significant portion of what they are here to do. This is necessary because otherwise the PC can flatly veto the outcome by both RAW and clear RAI. If Chejop is our big bad for the campaign, odds are
some PC has a Major or Defining Intimacy that states how much they loathe him for his villainy and won't let him get away (or just how much they hate to lose in general), so he just... can't push the "I win" button. It's not even an "I can get away" button if the Intimacy is something like "Tie (Chejop Kejak: He Must Be Tried For His Crimes)". This is the straightforward application of what can be vetoed and no interpretation beyond that.
Instead, the ST is more likely to take this to the players with something like... "I think it would be dramatic if he gets away here and uses this legendary technique to escape you once, but it's expended by him doing so. I have a really cool setpiece I'm building towards, if you'll let me run with this", which might get players to say "yeah, let's try that, then." Depending on players' patience (and willingness to allow this buildup to a better climax vs being expedient), this may also all be unacceptable, and instead of Breathing on the Black Mirror going off at all, we end up with "no option is taken, then", which means that at least Chejop and one PC (and probably most/all other characters on the field) suffer the Glimpse of Infinity that we don't know anything about yet. It has to have equal narrative weight to work, and that's something that isn't a ST-only call.
Only the ST is allowed to cede use of this to grant the stylist a stronger result than 'equivalent exchange', and there's limits on that, as well.