If you have a Dev edition device why not unlock your boot loader first since it will wipe everything just get it over with. Flash your twrp and make your stock back up. Then let twrp install root. Done. If you are worried about losing anything from unlocking the bootloader then back it up to your PC. If you want a complete stock set up of your device then you have stock fast boot images for your Dev edition device and don't have to worry about that.