[QUOTE=Shienvien]How great is an yth's armspan compared to its body, exactly? It isn't greater than their body length, or is it? It might be that I have been imagining them having somewhat relatively shorter armspans than they do have - at most 3 feet with this not-yet-full-size yth, not that much and barely the length of, for instance Aemoten's sword's blade and under the length of it's tip's trajectory - and therefore pictured it more locked in place in comparison to other things going on amongst other things...[/QUOTE]
Okay, here we go... The length of the arm of a fully grown yth is approximately three feet, as you presume, and would be less than that - probably closer to two and a half feet - with these adolescent specimens. Combined with the width of an yth's body - ten inches when fully grown, more like eight inches with adolescents - this brings the armspan of such an adolescent yth up to seventy-four inches, or 187.96 cm. Comparatively a representative measure of shoulder-width for an adult male human could be around 22.83 inches, or 58 cm. The yth is anchored in the center of the two outermost guardsmen's chests, meaning that only half the width of either guardsman is actually a factor. Presuming that the three guards lie shoulder-to-shoulder beside each other (the rightmost guard actually has his shoulder underneath the middle guard's shoulder - not a detail I mentioned, but one that existed in my head) this would result in a distance between the two anchored hands that was 58 cm (the middle guard) plus two times 29 cm (the two outmost guards) resulting in a distance of 116 cm or 45.67 inches. By my calculations that means the yth will have the freedom to move itself 22.84 inches or 71.96 cm between having the arm of either side stretched out all the way. Even presuming that the yth was positioned at the middle of its field of mobility at the time - which it sort of was - this would still allow it to move 11.42 inches to either side, which is three inches more than the width of its "torso"...
It would really depend on exactly how Aemoten aimed his strike, but by my estimations the yth should get away with losing the middle-left arm, sustaining severe wounds to the other two left arms, likely mostly in the shoulder-region, and some superficial injuries to its main body, but nothing fatal.