To be fair, Mir isn't exactly subtle normally in her desire to kill all humans, and there is someone guarding Mir who really doesn't want you to let her out and is willing to talk it over with you. Better than average odds that Tastiella will warn Setsuna off before she even reaches the sanctum of the Crescent Chronicle-in fact, that's what's about to happen with Kefka, though he may not listen and if he does spring Mir right now, well Canon has clearly jumped the shark before the first major boss battle of the first game of the series.

In which case, someone will have to fight Mir, and it turns out she's a big marshmellow when it comes to physical combat so Setsuna should have no problem handling her. She was once almost killed by a Pom of all things according to suplamental materials.