[Suicide Junkie]

Point defense can shoot at seekers and fighters automatically. The direct fire weapons are resolved instantly, and cannot be shot down.

Mesons vs APB differ in research costs, build costs, size and damage/kt. The major difference is that APBs increase in damage at shorter range, while meson bLasters are equally powerful at all ranges.

Massive shield depleters are designed to be one-shot weapons. The 1K is because there is not enough space in the summary to write out the full number.
1K means anything from 1000 to 1999 damage. If you check the component, you can see exactly how much it does.

For Stealth armor, it is the ECM ability that is one per ship effective. Adding more armor segments will increase the hitpoints of your ship, but there are other armors that do a better job of that anyways.

Ship and fleet training have the same effect, and stack with each other. A ship with +50% in both will have a +100% bonus to both attack and defense. Placing ships in trained fleets, even if alone, is a good idea.

Multiplex allows the ship to fire on multiple targets in the same turn. Whether your ship does or not and how to make it do so, is a question for the AI guys and Strategic Combat Strategists of the forum.