How do I thread rear derailleur cable through this adjuster?
They don't pull apart; the idea is that you're getting the couple mm of adjustment range you actually need, and the upside is it's an attempt at the easiest possible design to adjust on the fly while riding. If there's something physically blocking the hole for the cable, you could presumably clear it by poking a pin or needle through. Make sure the cable isn't actually getting stuck at the end of the housing though, either by the ferrule or by the end not being prepped properly.