SK autoattack seems to only hit the primary target, but most of the other melee classes autoattack hits the primary + the side mobs. Am I seeing this right, and is this intended?

^ Yes, it only attacks one target.
Crescent Cleave attacks adjacent mobs too, so maybe it's working as intended...

Maybe, but it seems super weak. My shaman has better damage because it's autoattack hits 3 things crescent cleave damage isn't enough to make up for the difference by a long shot, even with a ton of skill haste.