It's been a while since I thumbed through it, but IIRC, one of the 90's RPG's suggested that the older Romulan ships, from the Romulan War to "Balance of Terror," used an early version of the AQS warp core which was good enough to send a ship out to the mission, but, once you shut off the warp engine, the AQS would collapse and the ship was either expected to finish it's mission and die in the process (sort of a huge kamikaze philosophy) or else they were in it for the long haul home.
Even if the BOP is equipped with a "regular" TNG-type AQS, that may be sufficient to explain the discrepancy. If Scotty is specifically scanning for a M/AM-type engine, and not finding one, he might conclude that the impulse system is the only power source they have. Either he can't detect the AQS, or he's just scanning specifically for anti-matter, and doesn't bother doing a "broad" scan. (Would the sensors even report it? They might filter it out as an error, because how could a black hole be *inside* a ship?
(Or it could be that the Romulans used some other type of FTL propulsion at the time, rather than warp drive. They pretty much have to have some type of FTL drive, as others have mentioned above.)
If they had partial warp capability to get at Earth and attack and then had to crawl back home at sub-light... what if the Commander and the Centurion were in fact both veterans of the "Earth-Romulan conflict of one hundred years ago" and are still around thanks to time dilation from a high relativistic trip home?
But Romulans are basically transplanted Vulcans, who have a, what, 250 - 300 year lifespan? Both the Commander and Centurion could be veterans of the Earth-Romulan war even without invoking time dilation.
And if they attacked Earth during the war, and then tried to escape back to Romulus at STL speeds, I can't imagine FTL-capable Earth would just let them go on their way.
As for the Romulan Empire being just the one system... I dunno, maybe. I guess that kinda makes sense. But then, to me, the Commander's line about "It is good to see the stars of home" would seem weird if he had only been a few dozen AUs away...
Yeah, I never understood the "one system" argument, when we were shown a star map
in the episode itself showing that the Empire consists of a large number of stars. (I don't know how many of these stars would have planets, but presumably at least the one labelled Romii would.)