I flew Song a few weeks ago, and Zone 1 was rows 1-8 plus a few other rows; unfortunately, I don't remember the others, because I was in row 2 and only noticed my info. I was just an ordinary passenger, no special class or membership; row 2 was fabulous because I was one of the first on and first off. When they board (at least at LAX and Tampa), they use a door between rows 8 and 9, so most people getting on turned right, but I turned left and had few crowds to contend with. Getting off, they use a door ahead of row 1, so I was the second row off!
I was in an aisle seat both ways, and although the underseat space ahead of me was slightly less in width (maybe 2" narrower), it wasn't significant; also, because I boarded so quickly, I had no problem getting my carryon overhead, so it made no difference at all.