[wrxtasy]

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As a Kodi media player, Winner = WeTek Hub, due to stuff like 10bit 720/1080p HEVC, refresh switching, 24p, HD deinterlacing for OTA TV viewing, PCM, DTS and HD Audio all working properly in Kodi. Some of these Issues can be worked around on the Mi Box, see here.

Is HD deinterlacing not working properly on the Mi Box in generell or just in Kodi? If the Live TV Channels App together with a tvheadend server backend is deinterlacing well, then that would be good enough for me.

from that link above...no Hardware deinterlacing in Kodi...but...

• (22 Nov.) - Hardware deinterlacing now working for up to 1080i mpeg2 / H264 TV viewing when using the TvHeadend for Live Channels App. See HERE for install details.

Be aware TvHeadend Live Channels is exactly that "Live Channels" - there is no Recording, Timeshift or Timer functionality yet. Its TV streaming only from a backend server.

Is a USB-Ethernet Adapter good enough to stream full Blu-ray rips from a NAS?

I do not have an adapter to test.
802.11n WiFi seemed more than fast enough when I tested various clips, even 4K ones. If have very good WiFi tho. User results may vary.

Does the Mi Box support CEC for example to turn on the TV, when I power on the Mi Box and to turn it off, when I put the Mi Box to suspend. Can the Mi Box change TV's volume trough the Volume keys on the Mi Remote utilising CEC?

Very basic HDMI-CEC control from my TV remote works for GUI navigation.
The Mi Box Automatically goes into a Suspend Mode when I turn my TV OFF. And Resumes again when I turn the TV On.
The Remotes ON/OFF button either Suspends the Mi Box or Reboots it. It does not Turn Off the TV via HDMI-CEC.

I strongly suspect Xiaomi have cut corners here and have not implemented AMLogic's full monty HDMI CEC control options because HDMI-CEC is not configurable in Android Settings like you would normally see with AMLogic Android.