Hang a batten board to prevent tile slippage.
Tiling out of plumb walls.
How do we deal with that doing tile.
Check your floor for level and walls.
Do you stack the tile.
I hesitate to bring this point up but i think guess your walls are out of plumb because you capped the metal upturn flange of the tub with the cement board.
Measure the wall where you will place the tiles.
So you ve got that bottom row of tile.
If this is the case you could remove the first course of tile and cut the board above the flange punch the board in and screw it.
How to deal with out of plumb walls for tile.
A batten board helps you start your bottom most course or row of tiles.
Install the wall tile in a pyramid shape.
I know smaller tiles will not look good but if tiles were larger like would this help mask the problem.
If this is not the case then forget i brought it up.
Find the midline of the wall from ceiling to floor and mark it with a chalk line.