There’s a reason we’ve been calling them Abhumans. 
Not at all – the soul (“anima” or “psuche”) is the classical label for what distinguishes animate from non-animate things, but any materialist in any era would attribute that to material causes. Epicurus taught that the soul was made of atoms and the Stoics taught that it was corporeal and mortal (though not always dying at the exact same time as the body). In the early modern period on which XoR is modelled, there were plenty of materialists writing about the material soul.
I was remiss to leave them out. The oldest Gara’u focus on creatures native to the deep rainforests: tiger, bear, ape, giant river otter, and leopard. The youngest Gara’u, those of the savanna and desert, include catamount, cheetah, jackal, painted wolf, desert sheep, pygmy elephant, antelope, and buffalo.
I’ll leave the polydactyl debate to those who have a grasp on it.
Definitely not. There’s a common core, but loads of the rest vary by clan–as well as by specific useful borrowings from non-clan animals.
At the risk of inviting more snark from Laguz about cis genital obsessions: as you’re changing your whole body around, relocating, re-angling, and resizing the genitalia can take a toll on their reproductive function, and it takes a particularly significant additional amount of planning and work as you Change to make sure you’ve left room in your changed body to gestate a child and pass it through a working birth canal. A lot of Abhumans don’t bother, especially if they already have young.
Yes, some do change to make multiple births much more likely. But in general they have a much, much smaller population than the nations to the north.
Seracca only change themselves, not others. That said, if a Seracca never wanted to change themselves at all, they’d be seen as bizarre and immature, holding back from participation in the heart of society. They’d almost certainly be encouraged to go to the human kingdoms as an ambassador of some kind–a context where their refusal would be socially useful rather than eroding core norms.
