Time to fix the recovery issue.
twrp-20151227-EXPERIMENTAL-shieldtablet-m.img
This is labeled experimental for a reason. Yes, it boots on the new bootloader and seems to work okay.
But, it is built from the very latest TWRP code, which to my knowledge isn't stable yet. That being said, I've only seen a couple annoyances like after screen timeout, it won't come back. I also haven't tested backup and restore (cm-13.0 still doesn't boot, so I have nothing to gauge that with anyways). So, here there be dragons and all that jazz.
Another note: It seems something major changed with this new bootloader and kernel. They are no longer inter-compatible. Eg. L roms and recoveries will not boot on the new bootloader and vice-versa. I don't know if there will be a workaround or not, but for now keep in mind that if you want to go back to Android 5.1, you'll have to flash an older blob first, like the one in my CM thread.