I knew this sort of thing had been discussed before, but I didn't realise it was on this very thread - and almost a year to the day
, at that!
The concept of a single-system Romulan Empire is really attractive and works within the initial episode, Balance of Terror. The name of the war, the "Earth-Romulan conflict" also makes sense, since it was one solar system vs another (albeit with some allies here and there). No need to try and explain this away in terms of the Federation.
Unfortunately, that is where the theory runs out of good ground. Every later encounter seems to refute the notion - Commodore Stocker takes a "short cut" across the NZ to reach Starbase 10, an absurb concept (in interstella terms) if the RNZ merely surrounds a single star - you are effectively cutting through a solar system! Season 3 episodes have the Enterprise actually crossing to the other side of the RNZ, in one instance getting almost instantly surrounded by enemy vessels, in another spending several hours on a hitherto uncharted planet.
I have not seen the TAS episodes recently, but from earlier discussions they don't seem the help the "single-system" theory either.
It's great to see that the original script supported the "single system" Romulan Star Empire model. It matches well with the "Earth / Romulan War" remarks in the dialogue as being fought between two space faring species.
However, even by the time that the famous schematic
was produced, this idea seems to have dropped by the wayside - the RNZ is less of a circular permiter and more of a wobbly line between two superpowers - which is handy for future stories!