due to my illness, I'm unable to complete my work, but with current version twrp you can root & install busybox do other rooted things. Things you can't are install magisk modules & flash some files via twrp as it unable to mount internal storage.
In the current state, I'm unable to finish my work still uploading it for whoever wants to root their devices.
ur device bootloader must be unlocked before u going to flash twrp
after flashing recovery manually boot android to recovery do not use fastboot reboot, it takes 30 sec to boot into recovery, then press power = vol up 2-3 then u will see recovery asking password for decrypting just cancel & slide allow modification & then mount system & microsd then flash magisk from sd card, reboot system & then again boot to recovery do same thing this time flash busybox zip then u can use root all root apps.
I'm making tutorial video and will post on separate thread guide
https://drive.google.com/open?id=0B8WbEJ1ZXbsvM29oYjEzNEE1Xzg