I think it would shake out like this:
1. Arrow cover to pin down goblins
I think I'd call this Parley. You have leverage over the goblins -- (if they rush your friend, they'll get lit up). And, you want them to do something (stay back). Somehow you have to make the situation clear to the goblins, so you're either using well placed shots to demonstrate your intentions or your just yelling to each other (a similar scene happens in the Clint Eastwood movie Unforgiven). Either way what you're doing is meant to be intimidating, so Roll + CHA. On a 10, they get it and stay back. On a 7-9, you have to give some concrete assurance of your promise to kill them. So, I'd say that you have to spend one ammo for an especially impressive shot/series of shots to really make your threat sink in. On a fail, you waste the ammo AND they ignore you and charge.
2. Swing Bridge Kick Fall
This where the GM makes the text book tell them the requirements or consequences and ask move. You say, "sure, but the dude might grab you and pull you off the rope or dodge your kick and swing at you like a pinata -- want to try anyway?" If the PC goes for it, then do Defy Danger as usual with either STR or DEX depending on which danger you throw at him. (Or just let the PC kick the bad guy off the bridge -- you warned that there might be consequences, but that doesn't mean that the consequences MUST come to pass. I'm fond of the occasional automatic win).
3. Zombie Horde
I think this one is hard to answer -- it depends on your GM move and on the player's decision.
"There are numerous corpses shambling around between you and Bad Guy and the nearest one lunges at you WHAT DO YOU DO?" That's one thing.
"You are overwhelmed by zombies clutching at every part of your body and you cannot possibly kill them all before they devour you WHAT DO YOU DO?" That's another thing.
Either way, what happens next is in the hands of the player. They might make any number of moves.
But, assuming they push through with sheer determination -- knocking down zombies and trampling them. Then, yeah Defy Danger with STR makes sense to me.
In my view, the bottom line with all of this stuff is that your job is to make a GM move and their job is to make a player move (or decline to do so). You won't know which player move they are making until they make it. There isn't a "correct" player move. So, in your "use arrows to pin down the goblins" example -- I think that would be Parley (like I said) but only if that's what the player ACTUALLY SAYS. If the player just says, "I shoot at the goblins" then I'd call it Volley.
Zap,
A.