Muramasa is fine, since it's basically the same principle of Jack the Ripper expanded upon. AKA: Various people attributing to a single figure.

Honestly, this is on Vance to decide if he wants the extra GMing challenge of having to write up challenges for a character who can cut through any defense or make a new weapon to bypass any unplanned challenges. Since that's always a bundle of a thing. 

Also, this is Mad Enhancement D. "Strength and Endurance parameters are up. Language ability is simple. Continuing complex thoughts over long periods of time are difficult." 
Simply put, this mental interference thing isn't a thing. Did you pull it from Fate GO or something? Mad Enhancement doesn't make you immune to mental interference, you're usually just too insane for it to matter.