If you mirror the points may look a bit odd but like you said no little bits.The thing that I would check is where does each pattern finish on the tray edge If square it should look pretty much the same as the back wall you have already done.So visually mirror may look better in this instance.
Just make sure when you get to the corners you follow around otherwise it looks like crap.Start on the wall that is the feature(biggest wall or the one you see most)
Simetimes not centering is better because it wraps around corner better.I dry lay on the floor each time I do it easier than...
Cut with grinder then polish back with 100# grit diamond polishing pad on grinder comes out like a factory edge if done right.better than any saw cut wet or dry.
The lighter colour expoxies yellow over time (all brands, people will try and say otherwise but they are full of it.Its just what a true epoxy does). Nothing is perfect.
Clean your shower well rinse off properly after using hair conditioner dont use bar soap.Otherwise your silicon will go...
If the kitchen was lower you could bring it up level to match hall.But to level the kitchen up (which would be easy) you will then have the 25mm that you have raised the floor by to make it level at the entry to the hall.
1.So you either have to slope the last row of tiles down to meet hall way...
To much crap to go wrong on rubi and the two bars twist(as things wear) meaning the cut gets a bow in the middle so either doesnt cut straight or doesnt cut at all.Used rubi for 20yrs now using sigma got sick of trying to make the tx900 cut straight. Still use rubi ts50 for small tiles .Funny...
Why was he using quick set.
Did he mix 2 different adhesives (quick set with normal can give strange results)
Did he mix half set adhesive with more water because it has dried out (meaning the adhesives chemical reaction had already happened so what you end up with is chalk)
Take a pic from ceiling height so we can see at least 8 corners to see if there is size variation.
Those look like glazed porcelain or ceramic tiles not rectified porcelain.Often one corner will curl up or down when made with that type of tile.(pain the butt to get flat)and not been rectified...