Patching and Plastering - Backerboard on Plaster?

I am helping my son restore the bathroom in his 1930's Philadelphia rowhouse. To save the tiling enclosure around the tub we had to also save the plaster walls above. We have used lots of plaster washers and structo-lite to make the repairs. But there are LOTS of holes and cracks, and filling and skimming to finish doesn't seem like a great idea, especially in a moisture-rich area. I would like instead to cover the walls with a 1/4" drywall-type product. I would screw to studs and use adhesive as well. There is room at the edge of the tiles for this, and it would not only look good it would add strength and stability to the wall. But the only thing I have found in 1/4" with moisture resistance is backerboard, and I am specifically looking at USG Fiberock backerboard. They make a water resistant wall board, but in 1/2 and 5/8 only.

Questions:
1) is this a bad idea, and if so what is better?
2) if I go ahead, what do I use for joint filling?
3) one side is "smoothish" - will light sanding the board and primer make it smooth enough for semi-gloss paint?

I would float out the walls with a mix of strutolite and master of plaster basecoat.Use 36" wide mesh on the walls first you should be very happy with the results.good luck b