Minecraft – way to always find witch huts

You can use AMIDST to check, though it isn't always accurate. Witch Huts appears as "Witch" with a dark icon.

They are independent of difficulty settings. They are generated when the chunk is generated, like biome features. They don't "spawn" dynamically.

If AMIDST shows them correctly, I've tried a few seeds which shows that Witch Huts are rarer than Desert+Jungle Temples and NPC Villages. I would say two times rarer.

A bit source digging (1.4.5) shows that they should be generated in the same way as Desert/Jungle Temples as a "Scattered Feature". The fact that AMIDST shows more "Temples" than "Witches" could be due to the fact that "Temples" includes both Desert and Jungle Temples on both desert and jungle biomes, while "Witch Huts" only generates on swampland biome.

Part of the relevant code in StructureScatteredFeatureStart and ChunkProviderGenerate.populate.

Finding an ice biome is a rare event so there is no "easy" way of finding one, unless you find a seed which spawns you in that biome.

Alternatively, you can use AMIDST to find a biome you are looking for within an existing world, so you are not walking around aimlessly.