This is always an interesting problem of definition and separation. If all the things google claims are absolutely essential and inseparable as part of 'Android' the license could well be interpreted as requiring them to open source all of it. If, on the other hand, these are all separate pieces of software and not totally dependent to deliver the end 'experience' then that contradicts some of Google's legal arguments.

Has Google actually claimed the kernel is "essential and inseparable", or did they just do that wrt the closed-source Google apps, though?

I don't consider that part up to Google's discretion. It simply doesn't function without the kernel. The question is whether the Google apps are also essential and inseparable which would tie them to the kernel as part of a derivative work (according to this interpretation which may or may not hold up in court).

It won't function without a kernel, but will it not function without the Linux kernel? Since the Solaris days with lxrun all the way through present time with Windows 10's linux subsystem and SmartOS's lx-branded zones, you could run Linux binaries (and entire distros) on a system without the Linux kernel, with various amounts of support/breakage. Still, no GPL2 kernel needed.

I suspect that when a lawyer uses "essential and inseparable" they do not mean the same thing a technologist would mean. The GPL itself doesn't use that language, and anyway it's trivial to argue that a full OS distro isn't "based on" (the language of the GPL) the kernel itself.

The GPL issue is completely separate. The law regarding GPL and where is applies vs LGPL is well understood.

Not really, no. There's the FSF interpretation on one hand, but on the other there are companies like VMWare which happily put binary blobs into the Linux kernel and don't consider it a "derived work".

There are disagreements, but let's put it this way: If even the FSF would call it okay, then it's almost certainly okay. And the FSF will tell you that the kernel license doesn't infect user space.