The original space grey was just a way to use silver as a base after the black iphone 5's chipped to hell and back, I had one and those were prone to chipping because it was a coating.
The new space grey is how I would imagine it should look, old space grey was just silver with a wrong name.
Yep. They've continued refining colors, and the staggered release schedule + old models kept around as budget releases has always made a mess of the space grey being a consistent shade. But I don't see why anyone should care that much, as I'm not trying to color-match my electronics.
Doesn't help that these are all aluminum and glass products which will look different colors depending on the light anyhow. The rose gold model always looked more pink under normal daylight than it did in product shots, for instance.