I am not so far along (as you can see from the pictures) that I can't easily change to concrete board. Would I just run the concrete board out near to the point where I would stop the tile then but up drywall to that?. Then bring the kerdi out over this joint between concrete board and drywall then tile out just over the kerdi? I am not concerned with increased price, I just don't have much experience working with concrete board and I had read from so many people that kerdi over drywall worked well. I know you may disagree, but if you have faith that the Kerdi is 100% water proof, you can see why someone would think that the material behind the water proof membrane wouldn't need to be water proof.
BTW, I had also considered using the DensArmor but I didn't know how the Kerdi would stick to it. I was going to just get it for the entire bathroom but then I read it has a rough finish and requires a skim coat to make it look smooth. But if it was just going to be under the tile around the tub I guess it wouldn't matter.