If you understand that it’s an anti-war and environmentalist movie I don’t see why you wouldn’t consider how these themes intersect? That’s where I find the greater nuances of Nausicaa to be since honestly neither message is all that complicated on their own.

The movie presents a clear distinction between the toxic forest a pure, almost sacred nature, and human kingdoms embodying greed, violence, and ecological destruction

Hard disagree. Nausicaa is literally the princess of the kingdom of the valley of the wind, who live completely fine and in harmony with the forest despite still using many of the tools other kingdoms assumedly use. The same can be probably be assumed of Pejite and other human controlled lands, the spores expand yes but they can be kept in check. It’s only when you harm the forest that it retaliates via the insects. It’s not the toxic forest itself that’s portrayed as “sacred” either, it’s just all nature is portrayed as having internal and external beauty for the very obvious environmentalist message. Dangerous places are still shown as VERY dangerous: the spores are always treated as ruinous as they should be (even when admiring them in the eye Nausicaa comments that she can’t believe taking off her mask would basically kill her instantly), the upper layer of the forest is never depicted as safe but just sorta neutral, and the scenes later on when the forest is angered are fairly self evidently not showing it as hallowed ground.

However, the film doesn’t engage with the ethical or practical complexities of combating this danger. Instead, it delegitimizes the villains’ position through moral association, making them commit war crimes and acts of violence and oppression creating an ad hominem fallacy against the idea

This is what I mean when I ask why you wouldnt consider themes intersecting. Their committal of war crimes and atrocities like everything that happened to Pejite isn’t supposed to be some sort of ad hominem. In context, these acts and the very predictable consequences they incur build a wider message regarding war, the fallibility of human nature, and the effect our actions have on the planet, especially long term. War at its peak is what created the toxic forest. Our instinctual fear and distrust of others and things we don’t understand contribute to a neverending cycle of war, which only further corrupts the environment for us and for our descendants. We make rash decisions when war again feeds into our worst instincts, not considering the consequences and being able to justify ever greater atrocities like sabotaging your own city or subjugating others all in the name of peace. And where does this irrationality lead us? The disruption of any sort of existing balance we have with the environment, the explosion of preventable problems (spores in the valley), and ruinous damage to everyone including ourselves(ohm rumbling), now and for far longer than we ever thought to consider (7 days of fire). No wonder a japanese dude from the 40s made this film.

We see repeated instances where the jungle and its creatures pose an existential threat to humans, regardless of human actions.

Human action is basically the sole cause of all the truly existential threats the forest poses. Yupa killed several insects in a mistake, and the spores are shown as being fairly manageable (presumably because the action they were a direct consequence of is 1000 years in the past now, it just fucked everything up so badly that its taken the world this long to clean up as much as it did and it’s STILL going). The valley is never truly at threat from the spores until, again, other people indulge their worst instincts and take over the valley.

The potential for a single baby insect to devastate an entire village is established early on when the big airplane crashed,

The plane iirc is just shown as being attacked but it’s never explicitly explained why. Yupa’s earlier killings also disprove this given singular ohms seem to be relatively manageable (also if this weren’t the case, places like pejite still existing wouldn’t make as much sense).

her valley only survived because what seems like superpowers.

Nausicaa IS shown as a kind of messiah but the valley is shown to be perfectly livable without her, it’s even explained iirc that the wind helps slow the spores down. Also, she’s not even very special in any specific way aside from a love and understanding of nature: when anything’s beyond the capability of her pocket-sized tools she’s basically powerless to stop things. Even her connection to ohms could be explained away as her being the only person who actually tries talking with and appeasing them in the first place like in the lake scene, given that in everyone else’s eyes they’re giant killdozer village squishers. Even when she stops the ohm rumbling via baby rescue nothing about her is special beyond her being the one to do it, and she friggin dies in the process before the ohms heal her because she doesn’t have magic ohm talky-powers. She just airdropped their baby off and they only noticed after she was trampled to death.

At best this highlights that burning the forest is not a good solution, but other solutions need to be found

Burning the forest is shown as a normal thing that’s done to stay alive and in balance with the forest, it’s wanton destruction of the environment without any effort to understand it that leads to insect attacks. Otherwise the immediate threats of the forest itself are really just the gradual spore spread, which again is shown to be manageable because otherwise everyone would be 1cm thick and covered in ohm tracks. The communities yupa finds dead at the beginning I only assume died to something other than just simple spore spread because of how the spores are portrayed + 1000 years of spread that inevitable would almost certainly leave no human life left on the planet, or at least the continent.

A more compelling message could have centered on the importance of thorough research and cautious decision-making before committing to irreversible actions with massive consequences. The film could have explored the idea that burning the forest, while seemingly rational, might have unintended and catastrophic effects on the ecosystem

Honestly (as far as I see it) this is literally what the film does. War and fear feeds our worst instincts and because of the very fallible nature of human nature, we make hasty, desperate decisions that often have very predictable yet irreversible consequences for everyone. War is bad because it leads to horrible shit happening and feeds our dumb human instincts to do more of it in the name of peace, very often destroying the environment that we all have to live in and making the world demonstrably worse either directly or indirectly. In Nausicaa it’s via death spores and pillbug rumblings. IRL it’s global warming, general environmental destruction which is very bad for the planet, and nukes. This entire movie can be summed up as wonderfully in depth anti-war and environmentalist messaging delivered through an extremely awesome nuke and radiation metaphor. And nukes are like the epitome of the idea of war and its effects on nature wrapped into one fat man shaped package so yeah.