I have not seen the TAS episodes recently, but from earlier discussions they don't seem the help the "single-system" theory.
Since you mentioned that, I watched "Practical Joker" today (and just a few minutes ago it was BT on public TV) and found events rather "interesting" for lack of better words:
Captain's log, stardate 3183.3. Our present mission, a routine geological survey of type four asteroids, is nearing completion. The cruise has been uneventful and we are now approaching the final asteroid in our assigned sector, a full seventy two hours ahead of schedule.
No mentioning whatsoever that the Enterprise is close to the Neutral Zone (but they are surveying asteroids...hmmm). Then:
KIRK: I demand an explanation for this unprovoked attack.
COMMANDER [on viewscreen]: Unprovoked? My dear Captain, your ship trespassed into Romulan territory, in defiance of your own treaty. We had no choice but to defend ourselves.
KIRK: We came nowhere near Romulan territory. I deny all charges and plan to file a detailed complaint with the Federation.
COMMANDER [on viewscreen]: You seem to forget that invasion of Romulan territory is punishable by death.
WTF? No further explanation is given, the Enterprise doesn't seem to even attempt to contact Starfleet, escapes and the story takes its course. Later:
SULU: Course stabilizing at one one four mark twelve.
SPOCK: We're heading back to the Neutral Zone.
KIRK: And you can bet those Romulan ships will be gunning for us.
SPOCK: The Romulans are giving chase.
MCCOY: They must be furious to follow us this deep into Federation territory.
The way I see it, there only seem to be two explanations:
- The Enterprise is operating near the Neutral Zone but that area is distorting subspace communications. Kirk and crew are unaware that the Federation has just reached an agreement with the Romulans allowing these to expand their territory and the Enterprise happens to be accidentally in it (this would be most definitely "Hornblower Effect". The ships operated far from home and often captains were not aware what alliances or treaties had changed since their departure).
- The negotiations with the Romulans about extending their territory are taking place but that doesn't keep the Romulans from already seizing territory which is going to be theirs - and everything within it.