I disagree. All we know about the timeline strings is that they cover things in their physical life. Like the Greek Fates, where when someone dies the string of their life is cut, their string in Animorphs coils into nothingness, signifying death. But it says nothing that excludes the immaterial, the soul, which is not of the body. Animorphs never discussed religion outside of reference or sometimes when the characters discussed morality, but in doing so didn't say that there weren't things like the afterlife.
I agree with zaprowsdower as well. The space-time that recoiled back into nothingness was the time-line that the Ellimist altered to heal Rachel temporarily and tell/show her the things he did. Once that was over he simply restored the time-line thereby returning her to her death. It wasn't Rachel that slipped into nothingness, just the time-line. And as for Animorphs not being big on the afterlife thing? I'm not so sure. K.A deliberately left it up in the air. She's clever. There was the ambiguous case of the spiritual rite of Elfangor trying to ease/soothe Tobias when he was between life and death. Granted Ax said his people don't believe in it anymore, well Tobias didn't even know about it let alone believe in it, but it still happened. To me that was enough evidence that Elfangor is still existing somehow even though he is dead. Also, incorporeal essence (souls) is something that living beings in Animorphs possess and I believe that's what is left of Rachel now.
So, to answer the op's question, I think Rachel's soul is asleep or resting in a dormant state and might just be called into consciousness again if someone (living of course) with very strong ties with her calls out to her, kinda like Tobias and Elfangor. Of course, I'm just surmising here but I think Rachel's tormented soul just needs to rest, she deserves that much after all she has been through. May her soul rest in peace
Had to edit this post several times just to make my point clear
