Jack, right you are, except for a small detail: you can overblow almost all chalumeaux, real or so-called, to the twelfth, but some leave a gap. Additional holes, close to the mouthpiece, will normally produce notes of inferior quality and stability of intonation, plus problems of fingering. Modern clarinets are miracles of evolution, and never lost touch with folk music.

(Also, I doubt that the absence of keywork renders an instrument more "responsive".)