Is that heel one which can be replaced when it wears out?
Yes, it's an hourglass heel, so in theory I can drop it out and knit/graft another one in. I do all my socks that way, but so far haven't needed to replace any heels.
This is also how you ply using the Navajo method - which is weirdly satisfying and uses just the one thread looped, so tends to keep any shading etc in order.
Oh, I love Navaho-plying, it's a really soothing thing to do. They do talk about there being a weakness, in that if one of the singles wears through then the whole yarn can unravel - but there's no better way to keep colour blocky and it's a super effect.
I shall make a Navaho-plied pair somewhen - but would probably still do the heels in a plainer, stronger yarn, since that's the point that gets the most wear in my socks.
As to the twist being 2S-1Z in a Navaho ply....
I've maybe got this all wrong (not unusual!
) - but my understanding is that a Z twist looks like a Z whichever way up you look at it, and an S like an S whichever way up you look at it, so even though one of the plies in a 3-ply Navaho ply is upside-down to the other two, the twist is still in the same direction?