As for why the Zone isn't a perfect stretch of a circle/sphere, well, the Outposts aren't in a neat circle in this map.
The asteroids that the outposts are constructed inside of are part of a asteroid belt ouside to Rom/Rem's own orbit. There would be an entire circle of about eighty outposts completely around the Romulan star. The neutral zone itself is inside the orbits of the asteroids.
As Rom/Rem orbit during the course of the local year, they continuiously move past the line of outposts, in their larger, slower moving orbits.
The particular outposts attacked just happen to be in conjuction with the two planets at the time of the attack.