As of v2.6.0 (released this week), WinUAE has full MMU support, so Amix, etc., should all work now!
As to copyright, yes, it still technically is, but the current owner probably doesn't even realize they're the owner, and I doubt they would care if they did. And they'd need an army of lawyers to prove it. Besides, the source code is lost. Amix is a historical artifact, of interest only to historians and hobbyists like us.