Alright, we've managed to successfully farm the crimson key. After attempting the usual ordeal of just camping a particular biome (or even using the meteor head technique) with battle potions and water candle, I tried to think of what else would cause a large quantity of enemies to show up in a particular biome.
Then it came to me; the pirate invasions.
I stocked up on maps (even just one pygmy can handle all the wildlife that shows up with a battle potion and water candle, so you don't even have to really pay attention to it) and once we had about 7 invasions lined up, we cleaned up the world natural spawn point by flattening it out and plugging holes, cliffs and caverns where enemies might potentially spawn. Then we hosed it down with crimson juice from the clentaminator so the biome was in place, and started going after pirates.
That's when Darvi discovered that we could just sit underneath the platform proper to *completely* avoid danger, but the pirates weren't really much of a match for us anyways.
Seven pirate invasions and four blood moons (in a row!) later, we had several cutlasses, a couple platinum worth of dropped change, sailor hats, patches and pants, one lucky coin, and one goddamn crimson muthafuckin key (showed up on the very last invasion).
It works. It's still tedious as hell, but it works and it can potentially be automated to make life easier by using traps and/or pygmies. Just create the biome of choice at the world spawn point (easy enough to find by removing whatever bed you were using and then activating the mirror), and stock up on invasions. Pirates were the easiest to come by for me, and they have their own astronomical chance of dropping some interesting goodies of their own.
EDIT: Oh, and as far as ice goes, yeah you can get faster than hermes/lightning boots. But only if you DON'T have ice skates. If you grapple ahead of yourself and let go while it's pulling you (or just use a door launch), you'll start sliding along at far greater speed than you can actually run at. You'll also keep that speed longer if you don't try to run along with it; just stand still and you'll slide right along.