Those sockets would really annoy me- I'm miffed that the light switches at different ends of a room are out by this much! I would expect a much better job from a so called professional (I wouldn't except this from my husband who isn't). The gaping on sockets with bevelled tiles is a known issue but the gaps could have been filled. Again bleeding from dark grouts, but they should have advised that the tiles wouldn't work with the dark grout or used a tile sealer to prevent it. I've always drawn a tiling diagram, working out the best layout to avoid getting little bits of tile at the end, centralise on the wall or wrap around the corners.

Glass is very popular, can be made to any size/shape from a template, can come painted or even with a screen print on the back, its heat resistant so can fit behind a gas hob and its easy to clean. Your local glass company can give you an estimate based on the area to be covered.

Mirror splashbacks can make a small space look really cool between wall units and base units. Apart from the standard mirror silver you can get antiqued (is your kitchen style dictates this) or coloured glass such as copper/bronze. If you are not that keen at looking at yourself then a smoked grey is a great option for modern contemporary kitchens. Mirror splashbacks can be cut around sockets for that perfect look. Ensure behind your hob that you have toughend glass otherwise it could shatter with the heat from the hob. A simple upstand in the same material as your worktop say 75mm high is simple then use a washable paint for above for a simple look. Hope that helps

I have to confess it’s a nuisance when manufacturers produce worktops in one size and then upstands and splashbacks in another. If you are having the splashback all the way along the wall which will also go behind the hob you could have something different behind the hob which would give you the additional length you require. Usual hobs are 600 although some are 900mm. Perhaps have a contrast behind the hob? Premier Range have a webiste with some lovely options on. Bearing in mind you have chosen a neutral light grey you could successfully have anything with a hint of colour.