I don't know about Swain, but I have thoughts on Udyr.
Everyone seems to be taking the word "blood" literally, but I doubt that it's meant to be. I have a feeling that all it is is that Udyr has a strong connection to mystical animal spirits, while Vastaya probably also have a have a strong connection to mystical animal spirits, perhaps even a genetic or "blood" relationship. Which would mean that Udyr, while human, draws power from a source at least similar to what the Vastaya draw power from.
Rakan seems a little… upset at this, but I have a feeling that it's just because he's not the most educated person when it comes to Udyr's job and intentions. He says
It is from our blood your gifts came, and my hand that took it back.
I still believe that the spirits that give Udyr power do so willingly, but Rakan seems ignorant to this. Almost like he's been indoctrinated into a radical rebel group…