1. Z is definitely just someone who is straight up studying Ranboo because he is one of the Enderwalkers, meaning following him, watching his every move and his whereabouts, documenting; feel like they're definitely studying him from just a species perspective, and not a "this is a conscious person with thoughts and feelings type of deal", but more like a fucked up nature documentary
2. Because of endermen's inverted vision sandstone looks like cobble to them, hence the block choice for the Sky city; also I'm pretty sure Sky is what the End was supposed to be called originally so there's that
3. Theory time wow: the inverted images of both a normal enderman and uhm a farlander enderman from that one mod? which would be white with green eyes (this or the early endermen could be the reason for the right side) and the other? white with red eyes.
So my money, like before, is still on: the enderwalkers and Ranboo aren't really half and half, they're full endermen but just a different deviation or mutation, for lack of a better word, due to either natural progression or maybe a scientific experiment (successful/failed);
Full enderman would also explain why when Ranboo says his "other half" outloud we only hear ender, because, well, that "phenomenon", if true, would only exist in the End and noone but somebody from there or somebody studying it specifically would ever have the opportunity or even the possibility of knowing
This would also support "there is a story reason to why the End is closed off, not because of people getting op items, and not only because canonically noone is aware of its existance, but because something happened there and somebody either doesn't want people to know, or is trying to keep it shut and away from everyone"
This was my main guess before and now with this "discovery" prompted by the reawakening of my image inversion hiatus it definitely seems like an actual possibility.