Tldr gonna be staying at a hotel with free wifi but paid ethernet/wired internet. And my boxes wifi chip has always been flakey. My phone has USB internet tethering and I've used that to pipe free wifi to other devices before. My box is the ematic 4k ultra hd if it matters.
"maybe"
Yes it can be done, but the big unknown is whether your phone as a USB Ethernet adapter is supported by the Android TV. It will support some devices and not others. You'll find out when you plug it in.
You could also get a wired Ethernet adapter for the Android TV.
Almost certainly yes - because drivers for that ship in kernel and it requires manual action from manufacturer to disable it.
I don't think that's possible, as far as I know, the only way to tether the internet from the phone to the box is to turn the phone into a hotspot in settings.
That should work automatically. Your phone gets detected the same as a USB ethernet adapter would, which is supported on android TV boxes.
You shouldn't need to go to the hotel to test it. Standard plug and play from the phone to the box.