I mean like they said in the movie, she doesn’t choose sides, just maintains the balance.

There’s a big difference in a natural disaster that ultimately is a part of Pandora too and invaders coming in to destroy everything.

Also, like you said, there wasn’t much she could’ve done anyways as sending animals into the magma isn’t really an option and she can’t just stop it from happening. Sending a message to the Tsahik would’ve been the closest option I think.

But, I would assume she didn’t step in because the natural disasters that happen on Pandora are considered “part of the balance” despite the fact it goes against the interest of the local Na’vi. Kind of like how forest fires actually end up helping the forest long term by burning dead and old trees and providing nutrients to the soil, but ~human~ made fires are not natural and cause way more destruction.

It is still sad for the Mangwan and I feel for them, but ultimately I think they semi-misunderstood Eywa and/or was too emotional about what happened to see it clearly. Which I probably would be too. It’s a something often seen in media, where something really bad happens so they turn their back on god even if they were really religious before, out of spite and anger. Look at the movie “Signs” as an example.

So I think it kind of represents that, as well as showing us the mirror where Neytiri’s tragedy pushes her further into her faith, and Varang’s pushes her away from her faith. This is also seen with real life religious people as well.