you should be able to run plex server and emby server side by side on the same server.
unfortunately the xbox360 plex clients are deprecated and no longer work (as far as I know).
If you do not need truehd/atmos, you can use roku and/or appletv devices (or fire devices if you are really masochistic).
And while you can use plex offline temporarily, you will still need a plex account to sign in your devices.
see also @ [HowTo] Use Plex with No Internet
