sudo flasher -f -F firmware.bin -F emmc.bin --erase-user-data=secure -R (replace firmware and emmc with your region variants) that will take 15-30 mins depending of your usb ports speed, and will reset N9 to factory defaults, ie after that you should be able to set developer mode (as a downside you need to reinstall every application after that one). And remember to set the clock to current date before trying to enable developer mode, otherwise enable might fail.