The blow-through rules for damage to henchmen got me thinking about how it is applicable to bows and other non-firearm weapons. Is it possible and realistic for a thrown dagger to go right through a low-toughness NPC and damage/kill another? I'm cool with the melee and firearms aspect of the rule, but I'm really hesitant to allow bows and other projectile weapons to do such a thing in my campaign for sheer sake of snotling-kabob cheesiness. Anyone got any thoughts on the matter?

Like other posts have mentioned - just imagine the other henchmen doing such things as: stopping to help their fallen comrade, hiding behind a log or tall grass after they watch their cohort get whacked, or just plain running off the battlefield in panic.

Multiple henchmen casualties that are taken simultaneously do not have to be explained away as each of them having been killed or hit by the attack.

Another idea is to think of ammunition as "units" rather than single shots/arrows. Thus one attack might mean that several shots/arrows are fired. Sure, that might feel a bit weird with the reload rules and so on, but the trick is to remember that combat time is abstract. One round might very well be several minutes. Having to reload after firing one "unit" is just representative of all the reloading you have to do.

you could probably also just have them faint after they are sprayed with blood or something like that (stumble over a severed limb…)

The others have pretty much answered the question. A combination of both of the options usually is enough to explain why a "single" shot brought down multiple henchmen.

Though the ammo units option doesn't work if the attacker only has a single throwing knife or axe... in that case you fall back on the reaction of the henchman alone.

For example - the attacker throws an axe and takes down two snotlings.

"Your axe takes the snotling in the head, splitting its forehead wide open and spattering those near by in blood and what brain matter it had. With a scream, the one beside it drops its weapons and flees. One of the orcs at the back bellows 'Wot! Where you fink you're going' and with a single swipe from its cleaver separates the snotlings head from its shoulders."

You get the idea - one attack and two casualties.