The Great Norsehound Rebuttal Continues
If outsiders come to an isolated, xenophobic town and defeat initial attempts to keep them out, then are attacked again to attempt to stop them from carrying word to other outsiders, it does not follow that even the most zealous of xenophobes would necessarily throw babies at the outsiders as ammunition and block their paths with the bodies of the elderly. Similarly, it does not follow that the Kadeshi would either be able or judge it necessary to throw any civilian infrastructure they might have at the Kushan. A civilian habitat wouldn't necessarily even have the engines needed for a credible ramming or human shield gambit.
To extend the metaphor, if this isolated community was attacked repeatedly and then penetrated, they may pull many of their defenders to deal with the penetration, but still want to have some token force at least behind to watch the border for more coming. Especially if again there were non-combatants to protect from the hateful enemies. Maybe if they're cultlike enough even these survivors would commit mass suicide rather than face the unholy wrath the outsiders will bring after they do escape, but that's another matter.
To create a no-one returns reputation requires defeating all comers. It doesn't require being suicidal. Further, what if someone else tested their borders while they were throwing themselves whole hog at one intrusion? The fact that only part of their military meets the initial intrusion shows in fact that they are capable of measured response. The other needleships had to be doing something when the first one met the mothership. Where were they? I certainly don't know.
Again, assuming the absence of anything outside what we see is as big a leap as assuming that there is more. Though small compared to the whole galaxy, the nebula covers a lot of space. I'm not suggesting there are Kadeshi anywhere but the nebula.
It may very well be true that a destroyed Kadesh civilization is better for the canon. I'm not going to argue that. I'm certainly a kadeshi fanboi, but I welcome stories where they're dead and gone. But not one whit of on-screen evidence directly contradicts the opposite position, there is only inference. You don't need to retcon any lines where Kadeshi prisoners reveal that they spent their entire civilization, because there are no such lines. All you have to assume is that the universe is larger than what we see.
My Post Homeworld War Stuff
Unsuprisingly I've got some ideas for post HW1 Kadeshi. All my fanficing at this point feeds into the HW:@ backstory(mod project, link in sig, short version: Replace HW2 with a fan sequal, ignore all HW2 canon.). The rough shape is this: Kadeshi civilization did survive, there were other ships, so on and so forth. The escape of the exiles led them to believe Apocalyptic Badness was right around the corner, so they hunkered down for it. More hiding, more piety, drilling to fight the hordes of evil, whatnot. After the exiles got settled in they sent a few diplomatic expeditions to the nebula, to try to make contact with their bretheren/return surviving POWs, but they passed through without seeing a single ship, even when following intel provided by said POWs. So they gave up.
A few decades passed before a strain of Beast infection found purchase in the nebula. The Kadeshi thought it matched up alright with the apocalypse and treated it as such, but weren't perfect in fighting it. The infection was eventually traced back to the nebula by Beastslayer fleets, who rooted it out. Battlefield alliances and infection vaccine technology exchanges became the foot in the door of diplomacy, and a very tenative dialogue was maintained even after the beastslayers completed their work. Relations were pretty rocky, though. Most of the galaxy considers them murderous pirates, and rightly so, and they consider everyone else in the galaxy to probably be murderous demons, so open borders and free trade aren't exactly right around the corner. Kadeshi rejection of land offers on the homeworld insulted the Kushan, and Kushan rejection of attempts to impress Kadeshi religion on them was met likewise. Factions of both sides struggle over weather any communication should exist at all. It's a pretty tense situation, but a few cultural and technological exchanges have taken place, and the few POWs still surviving at that point did go back to something like home.
Kadeshi are not central antagonists or protagonists, but their presence is used by the story to broaden the universe a bit and reinforce some of its themes.
Who decided to call them needle ships anyway? They don't look much like needles. They look more like trees or spindles or something. In fact they look a lot like spindles. I wouldn't be surprised if that inspired their original design.