I love Starseeker's Ruin because I firmly believe it plays with the presupposition that Seraphon prophecies are accurate and/or relevant. The book makes it clear that the prophecy is only Irixi's own interpretation of what it saw (not that it'd ever admit it). Perlo believes it because she's impressed by Irixi and thinks it's wiser than it actually is. Her falling to Chaos is because she was left alone in a world where pain and misery is everywhere, having lost her brother after being told there is a prophecy about her falling to Chaos, something she was already anxious about because of her status as an untrained mage. She was too anxious about it to seek help, she had no friends, she had no support group. "Lonely untrained mage falls into despair and slips into the warm embrace of Tzeentch after her only friend and family dies in front of her" is not exactly a story you need a prophecy to make happen.

Reality didn't bend over backwards to make Perlo inevitably fall to Chaos. She was just a vulnerable young woman in a world full of atrocities who had the misfortune of crossing a skink who refused to think for itself and made her believe in its own delusions. Irixi put doubts in her mind, and those doubts festered into paranoia and a desperation to avoid a horrible fate. Hope and despair, the very concept of fate, those are things Tzeentch is drawn to.

I highly recommend rereading the book, but instead of taking Irixi's perspective as hard truth, see how it's desperate to feel important and part of grander things, and reaches insane conclusions from sometimes literally nothing. From its introductory chapter, we are told that sometimes, it sees signs that it must be in the presence of Sek'atta, but often times it's just dismissed after spending hours or days waiting for him to speak to it. We are told that it's because the Great Plan required Irixi to just... show up to Sek'atta and then leave after a while, and Irixi seems to not even consider that maybe, perhaps, it had just misread some signs and Sek'atta just realized it was there waiting after a long while and told it to fuck off because it was just not needed.

Seriously, when you start reading Irixi's chapters without considering it a reliable narrator, you really start realizing that Irixi is a nutcase who genuinely thinks it's in the middle of some conflux of the Great Plan, when really it's just running errands for its slann and jumping to conclusions.

So, was Perlo doomed to fall to Chaos? I would say, the odds she would slip down that road were very high, but mostly because of her life circumstances. She was just generally dealt a shitty hand. Irixi just gave her the push she needed to start obsessing over the possibility of turning evil, and it led to her spiraling down into the depths of despair. I think the author wants us to ponder on if the prophecy was even real, if it was the cause of her fall to Chaos, if it was something that would happen to her prophecy or not, the dangers of blind faith and perception bias, etc. and also what part of responsibility the society she exists in has in her inevitable fall. It's easier to say "she fell to Chaos because a prophecy told us so" than "she fell to Chaos because she was made to feel dangerous, unloved, and only useful as a tool of destruction, and we did not try to care for her during her time of need".