no matter what you do, it'll be fine as long as you have a single waterproofing layer, and that that is done right. One only, never two, so no to, let's say for example, a plastic sheet under CBU under Wedi.
That is all you have to keep in mind. A single water (vapor) barrier. Then, yes, you do need to make sure that the water (humidity, moisture, H20, steam, mist, vapor) that gets under the tile and seeps, drips, percolates, slips, downward due to the force of gravity, finds its way into the inside of the tub and not into the studs or cinderblock down at that tub rim level. That explains why the overlapping is necessary and why the little air gap is necessary too. Behind the last tiles there is a little void, emply space, at the bottom, so that water that is there has only air in contact with it and not anything that might wick it back up into the wall. (Whew!)
all the rest is less important, and not mission critical. Whether to fur, to screw, to glue, to attach this way or that, to leave plaster or to remove it, to build out to accommodate some other need, is all up to you. It is best decided by the person delegated to be there. He makes on the spot decisions because he can see what he is up against and he knows best. I leave it up to you.
Having said that, I need to remind you to read up on Wedi instead of jumping to conclusions or assuming things that may not be true. It is a true replacement for CBU (cementboard) and plasterboard, so a little extra Wedi or one thicker Wedi panel is all you need if you want to build out the wall. Have you ever cut CBU bfore? Do you know how much dust it makes? Do you know how bad that dust is? Silica dust, cement dust. When you get your hands on Wedi, you'll be glad I told you to look at it first before deciding things based on assumptions about what it is. Tactically speaking, you'll be fine with Wedi instead of mixing in some CBU. It is not serious. It's just practical, to use Wedi to thicken the wall if you want to thicken the wall.
Although i find it hard to believe that scraping off prewettened plaster is deemed hard work, I can agree with you that it's not a necessary step. Skip that step. You can thinset Wedi onto a rugged surface mix of bare cinderblock and old plaster. Or you can screw it on. Or both.
david