Ryzen & windows 8

Can you install/run it? Sure. Nothing stopping it being installed AFAIK.

You'll have to rely on Windows update for drivers, most likely - as Ryzen boards don't typically have driver support for OS's prior to Windows 10.
You may also lack some specific functionality (potentially XFR etc, depending on how its implemented, of course), but I don't know for sur.e

Can be done, but can't imagine why you'd want to?