[Android Apps and Games] buffal0b1ll:
After writing the post above, I decided to refine it and post it to G+. Here's the updated version with rationale for not using the SuperSu you mentioned.

ROOTED N PREVIEW WITH TWRP AND NO FORMAT DATA (STAY DECRYPTED)

As of this writing Chainfire's 2.69 root is still getting lots of "can't rw system" and "I got encrypted and lost data because I used the stock boot img" feedback. Here's 2 methods (personally tested on already decrypted Nexus 6 and Nexus 9) that will successfully root and busybox, allow write access to system, and keep data decrypted and in tact. Should work on any nexus that already has decrypted data partition (you may have to add/omit vendor.img, and radio to/from the instructions below depending on your device). Even if your data partition is currently encrypted, this theoretically should still work and you'd just stay encrypted (I can't guarantee you won't encounter prompts about encryption passwords in TWRP or other encryption related TWRP issues that aren't in the instructions below. If you are encrypted I advise you to remove PIN, fingerprint, pattern on lockscreen, and change security to swipe before you even start this).

Materials needed:

PC with fastboot and a USB cable for MTP file transfer

Android N Factory Image tgz download and prior knowledge of how to extract it and fastboot flash imgs individually

Nexus 6, 6p, 5x, or 9 with newest TWRP 3.0.0-x available installed (and oem allow bootloader unlock and usb debugging enabled in whatever ROM your coming from...make sure your backdoors are open)

phh Superuser nocrypt boot img OR Unofficial Supersu 2.68 (boot, supersu, busybox) zip

Working knowledge nexus bootloader, PC fastboot flashing img, TWRP (MTP, nandroid, wiping, flashing img and flashing zip)

Ability to Google and find for yourself the android n preview, phh no crypt, and unofficial supersu 2.68 files

My recipe (3/10) for installing n on nexus 6 while retaining TWRP recovery and decrypted data and not losing internal storage. Of course backing up nandroid, TiBU data, and whatever else you don't want to lose to PC is the first step

Download and extract the N tgz and the zip inside on a PC

Fastboot flash bootloader, reboot bootloader, fastboot flash radio, reboot bootloader

Boot TWRP recovery and wipe caches, system, data

Move system and phh nocrypt boot img to internal storage over MTP

Flash phh no crypt boot img (this installs modified no encrypt boot img and phh superuser)

Flash system img

Boot system (if TWRP asks you if you want to install root say NO)

Install phh superuser from playstore

Install Busybox on Rails

Check root and busybox with jrummy root check app

My recipe (3/13) for installing n on nexus 9 while retaining TWRP recovery and decrypted data and not losing internal storage. Of course backing up nandroid, TiBU data, and whatever else you don't want to lose to PC is the first step

Download and extract the N tgz and the zip inside on a PC

Fastboot flash bootloader, reboot bootloader

Boot TWRP recovery and wipe caches, system, data

Move system.img, vendor.img, and unofficial supersu 2.68.zip to internal storage over MTP

Flash system img

Flash vendor img

Flash unofficial supersu 2.68 (it installs modified no encrypt boot img, unofficial supersu, and busybox all at once)

Boot system (if TWRP asks you if you want to install root say NO)

Check root and busybox with jrummy root check app

If not rooted, reboot to recovery and flash unofficial supersu 2.68 again

Boot system (if TWRP asks you if you want to install root say NO)

Check root and busybox with jrummy root check app﻿