This is wild speculation here- but I think there is a good chance that Tier is going to turn out to be the product of a time loop. Wendy said that a lot of seeming inconsistencies could be explained if we didn't think of time linearly.
It might be possible Tier's father was Tyldak. It's conceivable that Tyldak had an affinity for animals- that could have been why he took up wings, in part- because he felt aligned with the hawks. On the other hand, Tier has grey eyes- that might be relevant. Doesn't Mender have grey eyes? Or am I crazy? Clearbrook does... Could eye color skip a generation? Maybe Pool grows up to be Tier's dad- maybe some of the elves are thrust back in time? Or maybe Kahvi did find the Go Backs after all and recognized one of them. She might have resented recognition in general- as it would have compelled her against her will. Or all these events might just have taken place in that 10,000 year gap. Maybe in that time period, Kahvi and Tyldak found the descendants of the Go Backs wandering the plains- having become the Plainsrunners. And maybe Kahvi recognized one of them- stayed to raise the baby for a while, then left... possibly to confront a danger she didn't want to expose the baby to. (There had to be a reason the whole tribe disappeared! Maybe something wiped them out?)
We already know, because of Skywise, that elves can remember events shortly after birth (maybe because they are essentially born a year and a three months old!). The fact that Tier remembers his early years is not unusual. But his memory might be distorted by his lack of understanding. Kahvi aiming a spear at him might have been her killing a snake that slithered near his head. Heck, maybe he called some dangerous animal to him! Or maybe he was seeing through the eyes of an animal she was about to kill (he can do that, right?). As Redhead pointed out- maybe the angry glances were a result of arguments she was having with the father. Who knows? Maybe the fight was about her leaving.
Whatever Tier's origin story turns out to be- I'm betting there is some kind of a twist.