And Mifune has even less leverage. Herriman has more than twice the mass of Mifune now that he's spawned chitinous armor all over his body, and it was never stated that Herriman went airborne. Since the angle of the blow wouldn't require (or even infer) that to be the case, it should be assumed that he didn't. 

Even if (and it is an if, since the speed at which Herriman would be thrown back is much, much slower than the speed of Mifune's impact) Herriman doesn't have the reflexes to turn that quickly, Mifune still doesn't have the leverage to turn him back so his chest is facing up (especially since you yourself specified his sideways position earlier in the post). The blow also happened on the side of the neck guard, so the exposed portion of the neck wouldn't be the jugular.

Basically, you can't use details in your favor (like what a 500 mph impact from a human-sized projectile would do), then ignore them when they don't suit you (like Herriman's position and mass and momentum and training). Your post reads like Herriman is simultaneously flat on his back and lying on his right side.