In that case, Spock should have said "They're left behind."
Umm, no. In this model, the hero ship isn't leaving them behind. They are voluntarily falling behind, realizing the hopelessness of a chase started too late.
The Enterprise came to them
...Which is an extremely demanding intercept situation calling for high speed from the defender. It would be easier if the Enterprise
were flying past them, in parallel, or even away from them.
Otherwise agreed that "simple impulse" could be a speed falling exactly in a sweet zone where it makes the BoP tactically clumsier than Kirk's ship outside certain ambush situations but gives her interstellar legs for strategic deployment nevertheless. But that sweet zone is highly dependent on the circumstances of the intercept; in real life, defenders suffering from a speed disadvantage, even a slight one, have generally been unable to compensate with numbers of deployment patterns - say, in the Battle of Britain, or Cold War jet interceptor scenarios.
I'd still say the better bet is to declare the ship that Scotty observed in "Balance of Terror" highly dissimilar from the ones operating in "The Deadly Years". Not structurally, not inherently, but simply because the respective ships were operating on different modes and thus confusing Scotty. Declaring all Romulan warp drive in all episodes and all (pre-TNG) eras as being this "simple impulse" stuff that's limited to some lowish warp factor isn't a practical way to proceed.
Back to the original topic... In "The Deadly Years" the BOP is also firing non-plasma torpedo weapons too. Could those additional ports be the "conventional" torpedo tubes and the ship carries mostly nuclear and energy torpedoes in addition to the main plasma weapon?
Makes good sense. The other obvious alternative is that the plasma mortar is throttleable, though: first you fire a big fuzzy shot that has high odds of hitting something, and thus slow down the enemy, and then you throttle the thing to narrower, more devastating shots that are more difficult to aim and wouldn't work as the first stage of an intercept.
But the third thing we should consider here is that we don't
really have evidence of a variety of weapons being in use in "The Deadly Years". We see the big ball of plasma hurl towards the ship initially ("BoT" footage), and we see small bolts slam to the saucer bottom later ("Errand of Mercy" footage), but "big" and "small" are arbitrary definitions here. For all we know, the thing coming towards the camera in "BoT" is just a small fireball of "EoM" type in an "objective" side view.
...After all, remember that the supposed Tholian death rays looked like huge, diffuse clouds, too, when coming directly towards the Enterprise
Would the sensors even report it?
Probably not. After all, stealth is likely to be the very reason
the Romulans use the AQS system. They are stealth-obsessed, so their powerplants would in all likelihood be optimized for the thing as well. There would have been no way for Scotty to see the AQS if it by default is an emissionless metal box.
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.)
But Romulus is a planet, not a star. Wouldn't the odds then be that Romii (or Rom II) is a planet as well?
The exciting chase from the destruction of the outpost to the edge of the neutral zone consumes most of the episode, and spans one grid square on that map. This accommodates both a map depicting stars (in which case the grid square is probably something like five lightyears, so that no projection assumption causes Romulus and Romii to be to close to each other) and a map depicting planets (in which case the grid square or two represents some tactically more viable distance between two outposts).
There are later episodes that indicate the Neutral Zone is thick enough to take some time to traverse at high warp, and that entire star systems can be located within the thickness of the Zone (rather than in the volume of space encased in the eggshell of the Zone); these would seem to dictate a star map rather than a planet map. But whether a Romulan Star Empire encased in a Zone several lightyears thick and covering at least dozens of those grid cubes would hold more than two inhabited Romulan planets remains open to debate.