the shape of the neutral zone --roughly linear--
Umm, far from linear. The map we see has the acutely curved Zone cradling the dots Romulus and Romii in a manner that would place a putative "central star of the system" only about three Romulus-Zone distances to the lower right of the map area in the minimum estimate. Even a maximum estimate would keep the scale reasonable, and would perhaps make it likelier for Romulus and Romii to be co-orbiting planets.
If Earth dictated the segregation of the galaxy to "ours" and "theirs", why bother placing the RNZ any farther out from the orbit of the Romulan homeworld and its twin than absolutely necessary? It would make good sense for the Romulus-Zone distance to be something like 1/3 AU, the scaling then leaving the Outposts orbiting reasonably close to each other.
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. They might be in a map drawn two weeks later, though - after all, they are moving... The definition of the Zone might not be "X kilometers from the local star", but more like "Y kilometers from the nearest guard tower", a definition the victor and dictator of the treaty would favor, as it would keep the Romulans humiliatingly on their toes while making astro-engineering of the fortress chain simpler for the Earth side.