As long as a single ship has warp gate - reinforce or warp gate - full on a planet, that's what is needed for the ai to be able to reinforce at that planet. Very simple. If they don't have either, they can't reinforce there. If they have warp gate - wave that just allows for waves, not reinforcements.
The warp gates themselves don't affect the amount or reinforcements or how frequently they come or anything of that sort. So killing the warp gate would prevent waves, but that's all.
Since th command station has warp gate - reinforce, of course, that means that reinforcements can happen as long as the ai controls the planet. I wouldn't call that "cheating" at all, Malibu Stacey, as you can do the same thing. You're able to plop down defenses and structures on a planet with nothing other than a command station. This is why killing the command station has value.
Regarding how reinforced a planet gets, that's entirely based on the number of guard posts plus the command station. The wormhole guard posts are indeed not new, they're just now visible. The levels of when the ai gets border aggression, etc, have been tweaked somewhat though.
To be clear, border aggression is just when some guys from the ai planet get up and decide to leave their guard posts and kill you. Thy come through your wormholes as normal, not warping into your planet or anything.
Hope that clears a few things up!