Hey @yaroze,
I a have been using the 4.14 kernel on the C201 for a couple weeks now and it's been stable. Another user reported the 4.16 kernel working but I have not tested it myself. I ran into a few extra snags compiling the 4.16 kernel which I haven't worked through yet, though I haven't spent time on it in a week or to because man compiling is slow on the C201.
My plan is to finish compiling the 4.16 kernel and also test the new 4.18 kernel.
You can downgrade from mainline to the 3.14 kernel simply by reversing the 4.14 to mainline steps in the guide. In my experience it's always good to have ChromeOS bootable as well in case it doesn't boot for some reason. At least then you can chroot in, install the old kernel packages, and dd the kernel image to the boot partition.