Not able to flash recovery on my Shield Tablet K1
The shield's bootloader got overwritten during the Marshmallow OTA, and it is no longer backward-compatible (Lollipop ROM won't boot on this bootloader, and older recoveries neither).

The reason why you can't manage to boot into recovery is because the bootloader is preventing you from doing it.

When using adb reboot recovery, Android has already booted up, and, as a consequence, overwritten the recovery with the stock one. That brings you to the stock "android lying on the floor" recovery.

A developper at XDA made an EXPERIMENTAL Build for the shield tablet and shield tablet k1. (See here). Please use this at your own risk. Also note that Chainfire's SuperSU doesn't work with it.

If you want to root your tablet, you are not forced to do it by recovery. Give a this shot.