Theory: Kiri is the Avatar, Spider is the Seed Bearer
My theory is that Kiri is an Avatar Eywa created for herself, while Spider is being prepared to carry a piece of Eywa back to Earth. After Grace died and became part of Eywa, Eywa used her Avatar to create Kiri, basically doing the Avatar program in reverse. Humans created Avatars to enter the Na'vi world, while Eywa created Kiri so she could exist as an individual within theirs.
I think this explains why Kiri has such a strange connection with Eywa. She can connect with Pandora more deeply than anyone else, but directly connecting to Eywa overwhelms her. The "firewall" could actually be protecting Kiri because she's basically trying to connect back into herself. When she finally sees Eywa in Fire and Ash, I think the figure she sees is a projection of herself, possibly an older Kiri. Kiri is Eywa's Avatar without realizing what she actually is.
I also think the Woodsprite Spider touches in The Way of Water is the same one Eywa later sends when he's dying. Woodsprites seem to act as Eywa's messengers, so that first touch may have allowed it to imprint on Spider and understand his biology. Later, Eywa guides Kiri into placing its living seed inside him. That's why the scientists find living mycelium inside Spider keeping him alive. Spider isn't just adapted to Pandora now—he's carrying a living seed of Eywa.
I think Eywa's plan is eventually for Spider to bring that seed to Earth. Moving billions of humans to Pandora isn't realistic, especially when travel takes years and humanity would eventually overwhelm Pandora's ecosystem anyway. Instead, Spider could plant Eywa's seed on Earth, allowing it to spread through Earth's remaining plant and fungal networks and rebuild the connection humanity lost. Earth wouldn't become Pandora; it would become a healthy, connected version of Earth, where humans could eventually connect with their world and ancestors like the Na'vi do.
Spider may even have to sacrifice himself to plant it, becoming the first human consciousness stored within Earth's new network. His entire life he's been caught between humanity and the Na'vi, but maybe his purpose was never to become Na'vi. Kiri is Eywa reaching into the world as an individual, while Spider is the bridge carrying her gift back to humanity. Humans came to Pandora trying to take what they needed to save Earth, but in the end Eywa willingly gives them a piece of salvation instead.