The Android client has just got a complete rewrite that's made it better than its iOS counterpart (I use both regularly). However, it's still in beta and only available to Plex Pass users until the final release.

There's also an app for Samsung Smart TVs that I use a lot.

Yes, there's an android and iOS app as well as a roku app. The Plex Media Server works on windows, mac, and linux.

Yes and it's really good. 599 but totally worth it. Also works as a remote for the HTPC client.