Not all destroyers are as bad as you would think. Just out of curiosity I tried setting a Karuna up against a Hecate. The Hecate took the Karuna down to 70% with a single shot from it's main beam, but that was the only shot it ever got out. Because two seconds later a full torpedo salvo impacted on the front of the Hecate taking out both beam cannons.
The side slasherbeam fared a bit better getting off two shots before being taken out. All in all the Karuna still had 58% hull left after it took down the Hecate.
I only remember the percentages of the last try, but I let that mission run three times, each time the Karuna won easily.
I though something alike would happen with an Orion, but for some reason the Karuna didn't stip away turrets fast enough (and began with the blob turrets), making it a rather short battle.
As far as the torpedos go my money would also be on the Karuna, unless the Titans have longer range torpedos. Where the Titans shot one torpedo at a time, the Karuna puts out salvos of them, making them much more likely to actually hit.
As for why the GTVA even uses fighters. Because they need something to stop the UEF fighters from stripping their warships bare of weapons. Sure they got some nice anti-fighter weapons, but those alone just don't cut it if the enemy fighers can concentrate fully on destroying turrets. Even if the GTVA fighters are just distracting the enemy it gives the warships valuable time to shoot them down.
That would be if the GTVA fighters were totally outclassed to uselessness. But while the GTVA fighters are inferiour, they are still far from useless I'd imagine. Otherwise the UEF pilots wouldn't be afraid of the Balor.
the UEF simply didn't have all the shivan wreckage to reverse engineer beam technology from
That piece doesn't make sense. The Lucifer was the only vessel in the great war that had beams. And there was not a single part of the Lucifer in GTVA space. The GTI cracked beam technology purely form scans of the Lucifer. Maybe the shivan wreckage from the secend shivan incursion helped improve the GTVA beams to their current (blue) strength, but they had beams before they ever got their hands on shivan ones.
If there was any wreckage from the great war that contained beam cannons it could only have been in Sol, since at least the front half of the Lucifer (were all beams are located, including the sidewards facing) came through the node before blowing up.