I’m 99% sure that the API always uses radians unless you’re explicitly interacting with a ui component.
I’ve got this link saved for myself, and I reference it from time to time when dealing with units in maya. Referencing a file converts its unit scale - #2 by tfox_TD
As for setting quaternion values directly: not that I know of. The way I understand it, animation curves each control a single floating point value. So there would be no way to set things that have multiple values without multiple anim curves.
The fact that this isn’t built into maya is really annoying 
At work, we’ve got a python plugin to make a mel command that lets us set multiple keys across multiple attrs in one go. Plugin to set keys on multiple attributes over multiple frames · GitHub