Firstly, Dust imbuing has been seen in the show. Cinder's dress is imbued with Fire Dust and could take a bunch of hits while remaining intact, and also allowed her to produce flames for her to manipulate
Secondly, when I say that it 'disorients enemies and causes muscle spasms', I mean the Grimm. Grimm rely on basic, near-primitive fighting styles that rely on momentum and power, both of which could be nullified with a good old muscle spasm. After all, you couldn't really scratch someone if your arm wasn't cooperating to you. And the matryoshka doll thing was more of a physical thing than a Semblance thing. It would not make sense for only the outermost matryoshka doll to ripple while the inner dolls remained untouched. For better clarification, I guess I'll just say it's the 'object he touches', while anything inside that object is affected to a lesser extent.
Thirdly.....I did not know who Beacon's Headmaster was. Thanks for telling me.
Lastly, I've edited it so Kin is a bit below average in terms of stamina, which counts as another weakness, if that is acceptable