After finishing it at long last, there's an even more obtuse mental crossover in the ending / in the Ellimist book, reminding me of something towards the end of Vernor Vinge's book Fire Upon the Deep, but both things are too spoileriffic to even muse about properly
A couple decades later I'm finally reading through the Animorphs and have made it within a few books of the end, and feel like this is the Maziani from Cherryh's Alliance & Union universe, but with polymorphing. Now THERE's a scary image
Signy Mallory but she's the grown-up Rachel or Cassie