I realize that, given enough thinking, you can extrapolate any rationalization for a weak point in a movie. But the problem is, the filmmakers didn't clearly answer those questions during the movie.
Consider for a second a theoretical movie: it consists of one shot of egg. 10 seconds into this shot, the egg disappears. The movie ends.
Now, you could find some rationalization for why the egg disappeared; perhaps it was just a hologram. Your rationalization may even be halfway plausible. But the fact remains that the filmmaker didn't go that extra step to convey a coherent, credible, original story. That's the problem with Avatar.
To address your points one by one:
1. Home territory: I believe it, but they should have had a scene where the N'avi come up with an actual battle plan that exploits their opponent's weaknesses. Remember in Star Wars, when the rebels meet before the attack on the Death Star, and Ackbar says that the Empire doesn't believe in the threat of a small strike force? That's how you tell a story.
2. The humans are vulnerable: This isn't supported by any of the shots or dialog in the movie. All we see the entire movie is an extremely well-equipped base.
3. The humans go soft for PR reasons: Again, this isn't supported by either the shots or the dialog of the movie. Instead, we have many scenes with the militaristic commander saying stock bellicose action movie dialog. We have a few shots of the corporate manager second-guessing himself after he orders the attack, but he doesn't take action and tell the commander to pull back.
4. The corporation is watched closely: Also not supported by any shots or dialog in the movie. There are no shots of the corporate managers talking to Earth, and Earth warning them about their actions. And most of the militaristic commander's dialog is to the effect of bombing them into submission.
This is what I mean about filmmaking laziness. Any scenes filling in the story points you've referred to would have made this a better movie. But the scenes just aren't there.