Yes I guarantee that you can downgrade from Official OTA to RC29. Like I said I did it yesterday morning with no problem. I have used that same .nbh file since I started rooting RC30. Did you try using the one I posted?
XDA Developers was founded by developers, for developers. It is now a valuable resource for people who want to make the most of their mobile devices, from customizing the look and feel to adding new functionality. Are you a developer?