You may have some issues with nitro over Dingotone. I know Tru-Oil over Dingotone works OK, but nitro can be very fussy.

As you're staining, rather than painting a solid colour, I'd have been tempted to cut that horn back slightly rater than fill it, as it will probably look very different with the stain on compared to the wood, whereas you really wouldn't notice if the top horn wasn't quite so large. I'd start staining on the horn and see what you think. Just do that small area. If they match - all well and good. If not, then you can still file/sand/Dremel the horn to make a slightly smaller version that will stain evenly.