For movies, if you buy it from one of the major movies studios that participate in Movies Anywhere, it is automatically added to your library in Amazon Prime, whatever Google is using these days, Vudu etc.
But to your other points.
2. If you can find a high quality rip
3. All the streaming services work on iOS, Android, Roku, AppleTV, Windows and Macs and whatever Google device that Google decide not to abandon this week
4. I had a Plex server and 1000Mbps u/d and it still wasn’t as reliable when I was on a plane, outside the country etc