Well today I developed their eventual answer. Ideally I'd name it after some other aquatic animal to keep the theme naming, but lacking a name, I've stuck on the Whirlpool Torpedo. This state of the art torpedo, also unmanned like the Mangrove Missile, is almost just as fast and maneuverable, but is very heavily armored (with a design that helps to deflect lasers, to boot), as heavily armored as a Hammerhead, even featuring a weapon on par with the laser weaponries of the Shark and Stingray. (Sharks have a weaker, but wider-area, laser weapon; Stingrays have a more powerful, but narrow, laser weapon; this laser is shaped slightly smaller than the Stingray's but is as weak as the Shark's.)
There are a few tradeoffs, however.
1: The torpedo is huge. It's about as long as the Swordfish is, slightly longer in fact, albeit slightly more narrow. Its huge size means that it's relatively easier to hit, and while the torpedo is super-durable and basically can't be shot down, it can be knocked off-course, and while it is good at speed/maneuvering and is just as much a smart-torpedo as the Mangrove, the deflections can be stacked to indefinitely keep torpedos from colliding with key targets.
2: The torpedo is almost as destructive in explosive discharge as the Mangrove, but is overall slightly weaker, especially since the detonation is in the back rather than the front of the torpedo. (The explosion of a Mangrove aims forward from the site of impact, penetrating hulls even if the torpedo doesn't. The explosion of the new torpedo actually aims the explosion backwards from the site of impact, which makes it less effective at hull breaching, but was a necessary sacrifice for the other advantages.) It can puncture through hulls with actually greater ease than a Mangrove due to it literally drilling through armor (which penetrates through shields, too!), but while that will utterly obliterate Interceptors and Hammerheads alike, for a Warship it needs to actually get a very good direct hit to deal lasting damage to a Warship.
3: Because it is so huge, it cannot be mass-produced the same way Mangrove Missiles can be. A Swordfish isn't able to carry so much as one, and a warship has a limited supply of them, able to carry a maximum of ~250-500 (each slot taking up the slot of a ship that would previously be a Swordfish), and while they can be over time restocked, in the course of a battle of attrition it is possible to run out of them.
However, the advantages of a smart missile built to survive everything the latest generation of interceptors and even strikers which can be thrown at them, while being an unmanned weapon, make it hugely effective as an augment (rather than replacement) to the Mangroves and Swordfishes, to help them more effectively combat interceptors and strikers, and thus, more effectively combat the enemy warships.
By this point in time, the side with the Hammerheads and Stingrays has learned from their blunders in strategy, catching up and evening the warfare across the board, but with this new advantage to their enemy, they in spite of having previously been state of the art, are now a bit behind. Their compensation for this is, frankly: Ace Pilots. So, so many of their pilots became just so good at piloting the Stingrays that as long as they didn't have the misfortune to be massacred in the surprise strategic strikes, they learned to master their craft to the extent that they're just. that. good.
And with time, these experienced veterans were better able to train the new recruits, too. With the benefit of lots of experience, and now facing comparably-strengthed enemy craft rather than having a clear technological edge, and with their opponents being mostly unmanned (and thus, largely unlearning, tho the advanced targeting system of the unmanned torpedoes is always being updated to be better and better and they do receive extra guidance from human pilots), they just learned as humans how to fight that much better.
So the war remains even.