What Android plus phones proves is you can get excellent performance and fantastic battery life from Linux and third party HW. This could and should be applied to Linux running on an ARM64 system but not sure why. Maybe economies of scales WRT investment on the phone driver side.
First of all the userspace is completely different, secondly Android throughout the years has been aggressively changing the ways background process work (in the context of Android activities, not bare bones UNIX), thus it isn't the same as GNU/Linux where anything goes.
No it's not and never will be. Google says every year that ChromeOS and Android are merging but it's not happening. They are just merging some components, e.g. the Bluetooth Stack. ChromeOS got a new design a few months ago so they are still putting work into it.