How do invasions work?

[warmachine]

Getting to the point of landing troops on an enemy planet is a very unreliable process. You bring enough ships, select the planet to invade, easily wipe out the opposition, get the victory message but find the naval combat screen doesn't show zero as the remaining opposition count. Huh? This prevents the invasion of course. How do I wipe out all opposition first time?

Once you successfully invade the first planet of a multi-planet system, attacks by new, enemy system ships against your new conquest prevent subsequent invasions, despite the fact that they're debris in very little time. I have been prevented from invading second or third New Orion worlds for about 30 turns because they were able to produce 1 to 4 system ships every turn to attack my conquered colony, despite having over 10 armadas of warships to smash them into component atoms. I eventually completed the genocide of the New Orions by giving the first Orion colony to someone else. Am I doing something wrong?

[Doomed]

Not definate about this, but if a planet has built more orbitals then it can field at one time (3 per planet/moon) then you have to wait till next combat round for them to become targets. This has the consequence of showing the enemy as having some forces remaining after the first combat round.

As for the new orion problem.
There is a known bug that will be fixed in the patch that means the random factor of who attacks/defends in a system does not work, so you always end up defending at the moment. You have already figured out the only work round at the moment.