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?
Proud owner of a T-Mobile HTC One M8
OS: InsertCoin 3.0.8 | 1.54.401.10 | Android 4.4.2 & Sense 6
Kernel: ElementalX 14.8
and don't forget to thank me if I have assisted you in any way