Indeed, the synopsis for the second one is the most interesting. It implies that Jake has some kind of plan to make it too costly for the humans to return, but it somehow also relies on human nature to keep it together (or not, since the plan unravels because of it). And seemingly it involves Jake and Neytiri going off planet as a part of this plan. I am really interested in seeing how this is going to turn out.
I always found it interesting to imagine what the most effective way would be for the humans to return to Pandora. Unless Jake has found a way to directly influence Eywa (which seems highly unlikely to me), I doubt that just his warriors can do much against the humans returning. How about high altitude bombing of the area around Hell's Gate, or other places? I think if the humans are serious about returning, they can pretty much do anything to get their foothold on Pandora back, but probably the most effective thing would be to have a weapon so devastating that its presence alone will act as a deterrence for any Na'vi attack.
Maybe that's actually also Jake's plan: some kind of bluff involving telling the humans that Pandora's biosphere will actively fight the humans this time. Then the bluff might be called when a human "ally" of Jake betrays him in order to be forgiven for being a traitor and be taken back into the fold. Jake and Neytiri going off planet might be related to an attempt to sabotage the star ships, but the whole idea seems a little far fetched to me, as it will be far easier for humans like Max and Norm to infiltrate instead. So I really cannot think of a reason why Jake and Neytiri would go off planet. Especially Neytiri; what could possibly motivate her to go into space to the most alien environment she has ever encountered? Or maybe their trip is accidental, like that they were looking for something in a landed shuttle, which then unexpectedly took off with them still aboard?
I do like to speculate
My thoughts on how to ensure the safety of Pandora were pretty brutal (human nature, and all) but ultimately, would be completely effective. I suspect only Jake and the loyal humans left on Pandora would be able to live with it - and they would have to be deliberately vague with the Na'vi when discussing it ("The threat is gone, that's all we need to think about")
Essentially, the RDA humans never make it home. Humans loyal to the Na'vi board the ISV before she can depart, kill anyone still awake and euthanise everyone in sleep. The laser sail is destroyed so there is no way to decelerate her. Ensure she is filled with fuel to absolute capacity. At this point, set an impact trajectory rendez-vous with Earth, and set her engines to a delayed burn (so there's time to escape the ship) that will run continuously until her fuel is completely spent, turning the ship into a relativistic kill vehicle. Impacting earth at her full interstellar cruise speed (210,000km/sec) would effectively turn the ISV into a civilization ending, extinction level impact. It's unlikely any sort human life would survive long term (isolated pockets might well survive the initial impact, but would likely quickly succumb to the environmental shifts), and it'd likely push Earth into a new, post-anthropocene geological epoch with completely new species, biosphere and atmosphere. Any humans on the moon and Mars would die off without any resupply from Earth.
Why such an idea failed? Betrayal by a supposedly loyal human, or more likely someone (maybe Jake himself) is inadvertently too specific about the full implications of the plan to the Na'vi, who despite not exactly being a big fans of the Sky People overall, aren't quite as coldly pragmatic to the concept of planetary scale genocide as the humans are, and stop it.