Tiling an uneven wall in the en-suite.........

Frankly, I'd line it with some thin ply first.

Or nail some shaped battens on the wall and glob plaster in between using them as a guide to get it flat. Smooth is not required but flat and even is.

I wouldn't dot and dab on tiles if I were you. Grout will either disperse into the gap behind, or crack and fall out.

or dob and dab a piece of plasterboard up on the wall first. No need to skim it tile straight on it.

If you want an attractive finish, as the nice tiles suggest, you should replaster or something to get the walls plumb. Tiles getting narrower or worse still thin strips added or the grout getting wider as you look up or down a corner looks very poor!

I wouldn't, in a bathroom. You want to ensure there's maximum stiction between the tile and wall in a "wet" area. Flop the tile against the wall, and it should be near impossible to pull the tile off without prising the edge up with a scraper.

Spot fixing will not give you the same adhesion.

I wouldn't try to true up the wall too much. If the tiles are applied correctly than wall trueness can be largely hidden by starting tiling in the corner opposite the door, where the eye line normally rests on going in.

You can lose a lot of the cut tile edges into the corners you 'can't see' from the doorway.

Cheers,

Paul.