The only things that can run Stadia at launch are the Chromecast Ultras that come with the Stadia bundles. This is because they will be running an updated firmware that will not have been rolled out to existing Chromecasts or cast enabled devices.

Source: https://ww.9to5google.com/2019/11/13/stadia-ama-assistant-chromecast-ultra-firmware/

I do remember hearing that Google does plan on bringing Stadia to Android TV devices at a later time. So you may just have to play the waiting game.

This isnt MiBox specific but this is the first thing that comes up when I google "stadia android tv" :

https://www.theverge.com/2019/9/16/20869185/google-stadia-android-tv-roadmap

not until google Releases the App for that box the hardware should be able to run Stadia as all the streaming is done at google servers .. if you have a chromecast you should be able to run it just fine .

I can't find answer anywhere have been looking for some confirmation ever since Stadia was announced.

Chromecast Ultra and Chromecast Built-in are different beasts. The built-in lacks some features and I'm not even sure if you can send 4k content to it.

Probably won't work tho.

No.