Just to confirm, you are using image from here to go back to stock and not the OTA image?
Once you've flashed with this image, you should then be able to sideload the beta using the beta OTA image from ...
Try using this tool.
Make sure you put the files on your primary hard-drive to avoid errors....
Bootloader is hold volume down with the device plugged into the PC. If you want to get into recovery, remove the USB cable as that causes it to skip recovery and hold volume up.