the new neutral zone would be redrawn along a mutual compromise between Romulus and the Federation for a list of worlds that neither side will try to annex.
And indeed ENT "These Are the Voyages" confirms that the RNZ was renegotiated as part of the Treaty of Algeron in 2311.
However, ST:NEM then indicates that the particular region of the RNZ shown in the "BoT" map remains unaltered in a late 24th century floor mosaic, for the relevant part. That is, the thickness of the RNZ relative to the dot-to-dot distances on the original map has not changed. All respect to preservation of ancient art and whatnot - but would it be plausible that the Romulan Senate would continue to feature artwork depicting a political situation that is significantly less advantageous to Romulus than the true, current one?
This sort of suggests that the dialogue-confirmed renegotiation did not actually alter the thickness of the RNZ, and that it could always have accommodated entire star systems even if it did not necessarily yet accommodate Devros in TOS.
flying around it (which they seem to do in "Face of the Enemy")
It seems to me the Romulans have a way of flying around it in episodes like "Tin Man", "The Next Phase" and "Paradise" and perhaps "Eye of the Needle" as well, since Romulan presence outside the RNZ goes unexplained there. But in "Face of the Enemy", Romulan penetration out of confinement is actually discussed, and Troi claims Tal'Shiar codes will make it possible to avoid detection within UFP territory. This is never put to a test, because the Romulans already meet the E-D before launching deeper into UFP space - so the indications are that the Romulans had previously successfully escaped from the RNZ into UFP space where the E-D now justifiably dwells, and that this was all due to nothing but good cloaking discipline.
The idea that the RNZ is made impenetrable by tachyon-beam sensors is thus actually refuted in the very episode that people generally attribute it to!
True, but then he wouldn't be standing there in the President's office giving legal advice while the Klingon ambassador barking about the extradition of Kirk. That would earn him at least one pointed "What the fuck are YOU doing here?" remark.
Hmm... Generally, visual communications don't reveal much of the surroundings. And in any case, the President would probably cut communications for the duration of consulting Sarek and Nanclus. After these consultations, the Klingon impatiently says he's waiting for the answer - a phrase consistent with various cases of temporarily cut communications.
That is neither here nor there, though, as Nanclus could quite plausibly use his diplomatic position as a cover for his presence, hiding his actual political role. There may well be Romulan diplomats, even ambassadors, in the Klingon Empire as well; that alone is no indication of a close alliance. Indeed, even the mutual hatred between the Cardassians and the Breen didn't stop the former from having an embassy on the homeworld of the latter (unless that was some sort of perverse humor by Dukat in "Indiscretion", considering the complications of the whole Breen homeworld thing).
What's significant is that nobody thinks it's odd for Nonclus to be in that office or to be present for that confidential briefing. Relations between the Federation and Romulus were never THAT good.
There's nothing odd as such about him being present when the Klingons call the President, though. The secret briefing in turn is secret. And that IMHO is well explained by the Romulans falsely providing aid to the Feds, and possibly to the Klingons, as part of their plot to ruin both.
Interestingly, ST:INS would have us believe that Romulans went from "thugs" to an "empire" around this time; since nothing much else is known to have changed about them, the "empireness" might come simply from them engaging in open diplomacy with their enemies for the first time.
Also interestingly, Kor would in "Blood Oath" boast of a victory against the Romulans in a timeframe preceding this movie; if there ever was an alliance between the Klingons and the Romulans, it would seem to have gone sour in the mid-2270s, making it plausible for Nanclus to bring messages of goodwill to the UFP in ST6.
Final intrigue comes from the fact that Nanclus and Sarek share a sash color and a seating sector at Khitomer. Also, the background colors of flags are telling: UFP flag corresponds to the blue UFP sash, Klingon flag (the Nazi-alluding red-white-black one is seen here for the first time, I think) corresponds to their red sash, while both the Romulan Bird of Prey and the Vulcan IDIC are seen on yellow background in flags and banners. Is there something going on with a political block involving all Vulcanoids, despite Vulcan's supposed continuing membership in the UFP?