Oh, a Kortul would most certainly "immediately" kill a 20-HP scout.

What that particular line in a weapon's entity block describes is how weapons apply damage, in relation to the effects. For example, beam and pulse-beam weapons apply "OVERTIME", whilst projectile-type and missile weapons apply "BACKLOADED".

The first is fairly obvious, whilst the second means that damage will be applied once all weapon effects have impacted the target; in the event that weapons effects don't impact the target, damage is still applied, as the engine has already determined whether or not a ship is damaged or destroyed the instant a weapon is fired at it.

It simply waits to show the effects.

Its only cosmetic, if your weapons kill a ship, that ship won't fire/use abilities etc., and yous ships will stop firing at it before it explodes.

I don't think it's that much of a nuisance, personally.

You can move a ship away and trigger a repair before the damage is applied and it will very possibly survive.  I.e. ships stop firing at the target, marking it as 'dead', but it repairs hullpoints first and ends up alive with the ships targeting something else.

That may explain some of the oddities I have seen in my development of the caps. Often they will be about 10 to 50 hp left then the AI will shift targets and the caps survive. I often have them sitting in a field of repairs and that may explain that idiosyncrasy.