Other suggestion I got from /r/ and XDA:

I tried fastboot boot TWRP or cyanogenmod recovery, no luck, bootloops.

I received the 6P from my father who passed, no idea where he got it (Best Buy, Google, Won in Pool?). But I called Nexus support (855) 836-3987, spent about 2 min in the phone tree and waiting to connect, likely less. Then 3 minutes with tech, explained the battery failures between 15% and 43%, phone untouched yesterday rebooted itself (assumed battery), bootloop nightmare.

No adb, no recovery. That I had tee’ed all the logs. At that point he said he would email me. Two minutes later a checklist of things to try, anyone reading @pocketables threads has already tried, and many of which we wouldn’t even have bothered with in the beginning:

If you have trouble installing a system update on your device, the following may help.

To check for system updates or resume download of a system update:

Plug your device into a power outlet or make sure its at least 50% charged.
Restart your device and connect to a Wi-Fi network.
Go to Settings > under “System,” touch About phone (or tablet) > touch System updates > touch Check now.
If you experience issues downloading or installing a system update, wipe the cache partition:

If you can access the settings menu in your device:
Go to Settings > under “Device,” touch Storage > touch Cached data > touch OK.
If you are not able to access the settings menu (such as when the device hangs when updating), use Recovery mode:
If your device is on, hold the Power button to turn the device off.
When the device turns off, press & hold the Volume Down button, and then press & hold the Power button simultaneously until the device turns on. You’ll see the word “Start” with an arrow drawn around it.
Press the Volume Down button twice to highlight Recovery mode.
Press the Power button to start Recovery mode. You’ll see an image of an Android robot with a red exclamation mark and the words “No command.”
While holding down the Power button, press the Volume Up button.
Use the volume buttons to scroll to “wipe cache partition” then press the Power button to select it.
When the cache is cleared, press the Power button on “reboot system now.”
If you still have issues downloading or installing a system update, reset your device to factory settings:

Resetting your device to factory settings will erase all local data on your device. If able, backup of your personal local data (pictures, music and video files) is recommended.

If you can access the settings menu in your device:
Go to Settings > under “Personal,” touch Backup & reset > touch Factory data reset > touch Reset phone (or tablet) > enter your Pattern, PIN, or Password if prompted > touch Erase everything to confirm.
If you are not able to access the settings menu (such as when the device hangs when updating), use Recovery mode:
Follow the 7 steps above for wiping the cache partition in Recovery mode, except choose and confirm “wipe data/factory reset” on step 6.
If you have any further issues, please let me know.

Replied to email with my serial (you can get it from fastboot devices) and a TL;DR; skipped the it boots part, got to boot to recovery, and explained what it was doing then put in a horizontal line, and pasted about 8 pages of log files where I:

  1. tried unlock & lock (got corrupt, unlocked to flash)
  2. tried flashing TWRP (no error, no recovery on reboot)
  3. flashed AOSP recovery (no error, no recovery on reboot)
  4. flash-all from [developers.google/android/images] (no recovery, no boot, boot loop)
  5. manual flash of AOSP [r/Android/comments/49lcfb/psa_when_updating_nexus_devices_via_fastboot_make]
  6. android, update all sdk and platform tools
  7. try above again

Sent email. Will let you know what I hear. Questions going around:
Was it running hot? Yes
Case: Yes bumper case no screen protection
Reason for heat? Unknown, I kept checking load thinking something must be going nuts, but it never was.

And as I said random battery shutdown between 43% and 15% battery, on marshmallow. NOT Nougat!

Anyone else having luck with RMA?
Turned down?
Leave us your experience. Even if you are moving on to newer a SoC.