AFAIK:
Fighters
1. Yes, they damage all the "physical" hitpoints of the fighter until it is destroyed.
2. Null-space will skip shields, but since armour is just physical hitpoints, and units retain all functionality until 100% damage is reached, armour is effectively not skipped.
eg. Fighter has cockpit, engines, weapons adding up to 9 hit points, armour adds 4, shields add 4 plus 20 shield points. PPB hits will take out the 9+4+4=17 hit points, and not touch the shields. Once the 17 is gone, the fighter is destroyed. Null-space will take out the 9+4=13 from the regular components, then the 4 from the armour, making it 17 hitpoints again to destroy. So there isn't really a difference between the two weapons when it comes to fighters.
Weapon Platforms
1. No, shield points act as regular hit points (someone please correct if I'm wrong, haven't played in a while
)
2. No, see above. All hit points are pooled together.
3. Yes, once all the hit points are depleted, the platform is destroyed and the next unit in the queue is targeted.
Satellites
1. Arkcon?
2. IIRC, they do. Again, I haven't played in a while, so don't hold me to that.
Basically, units are just extremely simplified in the game, with the space-based ones having slightly more detail. On planets, shields are just regular hit points, while in space, they act like shields. Damage is dealt in a queue, so basically everyone lines up single file, and you stand in front and shoot until either you're dead, they're dead, or the combat turn limit has elapsed.