My Odroid-C2 runs a mainline kernel. I use it headless, so I'm not sure about graphics support, but its been otherwise extremely stable. It did take a year or two after release to get full upstream support. I'd imagine this new board will follow a similar trajectory.

I don't think the RPi runs an entirely mainline kernel either.

The answer appears to be yes, with some exceptions like the integrated video encoder: http://linux-meson.com/doku.php

But, I dont have one to test - it certainly might ship with a weird hacked up bootloader or something. I do have a board based on an earlier generation Amlogic SoC that I'm quite happy with. Worked well out of the box and has continued to do so.

Maybe not right now, but Baylibre is working on this: https://twitter.com/Superna9999/status/1253307218688081920