Both of your examples sound just fine to me, but there's something really important I wanna stress: just because you're shooting at an enemy's back doesn't always mean automatic success, nor does it mean using Called Shot over Volley. The GM should decide each time, according the particular circumstances. Is there any form of danger going on? Sure, the guy's back is turned and he's running away, but are you chasing him? Maybe you could trip and fall, that's a danger! Maybe the guy is dodging and weaving and trying to lose you in a forest. Maybe anything! Every circumstance is unique and can lead to different interpretations!
You know how in D&D, if you're Flanking then you get +2? No matter what, always? Dungeon World isn't like this. Dungeon World wants you to look at the particulars of the situation and say, "yes, this makes sense" or "no, this doesn't make sense." For example, if you're flanking Bob the Useless, then one of you is probably going to just deal your damage, no roll necessary (because that dude is useless, geez). But if you're flanking Bill the Master Swordsman? He doesn't care about your dumb flanking manuever and he can fight both of you at once, no problem. He's a master swordsman, that's child's play! The GM gets to decide if an enemy is "helpless or surprised," always, based on what he knows about the enemy and situation.
So, bottom line: Did the decisions make sense to the whole table? If everyone nodded, like "yeah, that seems right" then you guys made the correct decision, no matter what it was! If half the table was like "that seems weird to me..." then you made the wrong decision. Easy as that!