5x8 Bathroom Remodel
ARI001, correct me if I'm wrong, but I believe the code you refered to applies to greenboard not cement board. A moisture barrier is needed either behind the cement board or on top of it but not both. (creates a moisture sandwich) If you are furring out the walls so the flange is flush with the furring material, then you could use 6 mil plastic over the furring material and down over the flange. This way the cement board can be installed over the flange and remain flat. The width that you have can be gradually tapered with furring strips of less thickness the futher you get away from the tub. Or you can install the cement board above the flange, silicone between the flange and tub, then mesh tape and thinset the bottom edge so it remains flat. Thinset, tape the corners and butt joints. Before tiling Redgard the cement board.
Or the other option is fur it out,(forget the plastic) install the cement board over the flange and then Redgard. I have never used roofing cement anywhere in a bathroom. I believe your tub has a short thick flange, so over the top may work out best for you. Attach the tub as ARI001 suggested. When insulating use unfaced batts, or cut the paper of faced batts.
Work is for those who can not fish.