Going back to your tiling - here's my work in progress - set a batten exactly horizontal to work from. Once all set, you can fit the bottom tiles. Done something here I virtually never do - use full tiles into the corner - risky because it's so easy either for the gap to close resulting in having to try and cut slithers or opening up causing a gap. I'm only doing it here because the tile boards have been installed absolutely plumb.
I read that when tiling a floor you start in the middle, how much of a difference does this make? Seems to me that means having to cut tiles both sides, rather than if you start at the edge and work across? I take it you're talking about the wall? You recommend not tiling up to the corner?
Whenever I tile a floor I set two rows at absolute right angles to each other and then work from there. Often dodgy starting from a wall edge - they are often not straight! It's best to always plan to cut to the edges - rooms are seldom perfectly square and this allows you to adjust the edges and corners to suit. Starting with whole tiles is risky because if the corner isn't absolutely square that whole tile easily becomes a whole tile and a slither or you have to try and cut a slither off it. It's far easier to plan to have a 1/2 or 2/3 tile cut into a corner. As I say, I am only doing it here because I have already set the backing boards perfectly perpendicular and I know I can get away with it. Planning is everything - use your laser to work out where all the edges will be - around windows and doors, in corners. The trick is to try and avoid slithers which always look bad.
If you need flexible adhesive you'll want flexible grout as well! I don't know if you can get flexible grout... But you can certainly get an additive that makes it flexible! Cando.