We went with an Americh Turo tub. It was the only one that was 18 inches high, instead of the 20-22 typical of tubs that have the whirlpool features. This tub can also be ordered with a whirlpool, but I didn't look into that since we wanted a plain tub. So far, it's working out well as a tub and shower combo. However, it does feel deep if it were filled to capacity - I may worry if I had small children (but, I don't have kids, so I'm not sure at what age I would feel comfortable).

once you install the cement board (so that it sits above the tub flange), fill in that channel with thinset, and let it harden. once hard, go ahead and install your Kerdi, so that it's sitting no more than 1/8" off the tub. Then you can seal it with Kerdi-fix. That's the "book" explanation. If it were me, I'd use the same color siliconized latex caulking as my grout and seal it with that. Reason being that then you wouldn't have to worry about the color of the Kerdi-fix sticking out from under the tile.

I don't see a photo. So to start with I'll ask was the tub installed in accordance with the manufacturer's installation instructions? Most instructions include how the tub should be secured, and how the tub flange should relate to the tile backer board hung on the walls. With that out of the way... My "blind" response? If the cement board is proud of the tub flange, no worries. The bottom course of wall tile will cover the tub flange. Typically the bottom course of wall tile will be held above the tub deck by the with of a grout line, and that gap will later be filled with a bead of caulk. Often times the caulk can be color- and texture-matched to the grout. If the tub flange is proud of the cement board, yes, that can be an issue. If it just gets tiled over, the protruding flange can cause the bottom course of wall tile to kick out of plane with the tile above. It'll skew the tiles themselves plus it'll skew the vertical linearity of the tile in the wall-to-wall corners. If there is any movement in the tub, from loading with water, from bathers, any movement in the tub can cause friction squeaks because of differential movement with the tile being in contact with the flange. If the tub installation is fine, the existing cement board can be removed. 1/4" rips (or whatever thickness is needed) from a 2x4 can be fastened to the edge of each stud to pack out the framing, then the cement board can be reinstalled over the packed out studs so the cement board properly clears the tub flange. If the tub does not have to be reset, that is my preferred solution. An alternative would be to thinset and screw a second layer of cement board over the existing cement board. The thinset is there to fill any voids between the two layer to prevent differential movement. The long screws are what will fasten the new cement board to the studs. Regardless, make sure whatever waterproofing that is to be used on the walls is reconsidered and adjusted if required.

Not a silly question. When you have water forcefully hitting the inclined side of the tub high enough up, though most of it will splash downwards, some of it goes sideways, and some of it -- counterintuitively -- splashes back upwards from where it strikes the side and you can actually have a small puddle start to form on the floor in this area from water spray bouncing up and escaping this way. It the water doesn't hit the side of the tub, but only the bottom of the tub, it doesn't bounce high enough to do this.