tiled upstands

[Burnz0]

Hi All. I've just had someone fit the floor tiles in my bathroom and due to some pipes that need to go in the wall still that were in the way I'm going to be left to do the upstand tiles myself. I'm not overly happy with the job he's done as they're not particularly level but I'm going to live with it for now and ndo the best I can. He's left me with 10cm cut skirting tiles and I'm hoping for some advice on fitting them.
Firstly, should I fit them level on the wall or parallel with the floor tile? Secondly, do I leave a gap or place the tile directly on top of the floor tiles? I've read I should leave an expansion gap of x amount? It makes sense to me to level them but this will mean that the gap between the floor and the wall will vary as the floor slopes. Doing it the other way it'll be more noticeable to the eye if the skirting isn't level as it will be on the wall rather than the floor. If for example I leave a 1/8" expansion gap this might increase to over 1/2" in places so how much can be hidden with the grout (or silicone) to fill in the gap?
Finally, the wall hasn't been plastered right to the floor in places and it's also slightly out of plumb. Do I need to fill out any gaps before or can I just put more adhesive in these places? I was told to back butter the skirting but doing this will mean there will be voids behind the tile at certain points. My instinct is it probably won't matter too much either way as the tiles are right near the floor and supported by the grout/silicone anyway so shouldn't fall off.

[Crooksey]

Run the skirting level, the height from the floor will be absorbed by the silicon bead.