[Nexus 6P Android Development] Kapak13:
A lot of people were reporting that 3.0.2-2 would fail to complete a backup, if yours didn't fail then you might be OK.

As far as the twrp 3.0.2-3 goes, I'm unfamiliar... You're the first source I've heard of it from. It may be out by now but I'm just not personally aware... And again, if this was the version you backed up with, you could be OK.

Chances are slim of having any irreparable damage to your device, but it might be best to flash back to 3.0.2-0, but use it only for Marshmallow flashing.

I feel uneasy giving instructions without others chiming in to correct me since everyone can have a completely different outcome doing the exact same methods. And its hard to know what is happening without your device in my hands.

If you run into any issues, there are many other forums around here on xda with many very helpful people who can get you back on the right track. I do not claim yo be an expert, chances are I'm just dumb lucky. But the method I shared worked for me.

From what I've read and heard, its best to use 3.0.2-0 to backup your EFS then copy that backup to your PC for safe keeping. Another user I talk with a lot had the EFS issue and had to get his phone replaced, while I think some others had some sort of method to restore. All of this stuff is something I haven't dealt with personally and don't know much about.

I truly wish you the best of luck in your journey and I hope to only hear your success in reverting back.

But just so you can compare and know what I have on my phone-

Flashing from M to N I wiped everything except internal, updated twrp from 3.0.2-0 to 3.0.2-1 then updated my radio to 3.72 (shouldn't matter for you) and updated my boot loader to the latest (can't remember the exact one.. I think 3.58? I feel that's the one.. Again, this might not be necessary.)

I then flashed digital high nougat and chose to come back to M by wiping everything except internal, then downgraded twrp back to 3.0.2-0 then flashed the mtc20f vendor, then flashed resurrection remix, then flashed open gapps pico, I personally then chose to flash xposed (not at all necessary, only if you want it) and rebooted and everything worked fine.

Then I tried weta nougat by doing the exact same things as before and then downgraded once again exactly as before. I may have just been lucky, but one or two other people used this method with success.

Please update me with how things go, I am willing to work with you. I will even get in contact with others to find answers.

I am going to research this 3.0.2-3 twrp and see what's up. I'll try to get back to you asap, but just know that I leave for a flight in the morning (actually in just a few hours) and won't be able to get back to you for maybe a day or two at most.

Good luck and god speed my friend!

Last little tidbit, since I've read so much about difficulties with N and reverting and whatnot, I am personally choosing to stick with where I am now (Resurrection remix 9/7/2016 release) until more N Roms come out with a sure fire easy way to flash. I'm banking on some Roms like PureNexus (will soon be called PureExperience) and Beanstalk (just to quickly name a few) to have the tried and true flashing method we all know.

Update- I couldn't find 3.0.2-3 on the official twrp site.. But found it on some index site? I back tracked the index as far as I could go and it was full of random stuff I personally wouldn't trust since I'm not familiar with it...