Depends on what type of shower you prefer, if you like a good needle like strong shower flow on your skin go for round, if you like a feeling of being in the rain go for the square. The smaller the shower head the stronger the flow, with the exception of the wand like shower heads and telephone type of square shower heads, they are weak. We went for a large square shower head in our new home build. Also don't forget the water pressure in your house as that is another factor to take into account, if it is weak then you will have a trickle!

We have recently updated our ensuite and main bathroom to have tiled base showers. Our ensuite is alcove and we installed a hinged door frameless shower as 1200mm. Our main bathroom shower is a single panel about 1000mm with an overhead shower rose extending 450mm. Most water is contained but there is a little splash forward onto the drying off area, so I imagine there would definitely be mopping up required from a 600mm screen. Perhaps you might do better installing a curtain, but since the screen is measured up and ordered after all the other work/tiling is done you can make this decision later (after using the screen without a screen for a bit)

You will need to re waterproof the whole lot if you try to take it off and replumb. There is nothing wrong with it functionally and the picture you have is just a picture of how it can look like not on how it should look. Placement is personal preference or at the installers discretion if you didn't specify. IMHO it looks good as they are both at the same height and it looks better , also not a fan of doing or having what is the "norm" or what everyone else has.

"this could be from some of the missing grout between the tiles on the top of the hob" Grout is porous, at least the typical old-fashioned sort is. I believe epoxy grout is waterproof, but ensuring there are no gaps between each tile and the surrounding grout would be a big ask. So regardless, what waterproofs a shower is the waterproofing layer that is underneath the tiling/grout. Moisture meters work, but depend on the quality of the interpretion. The 'expert' who failed to diagnose our shower leak had a moisture meter. Where do you see the water outside the shower? Did the first guy use his moisture meter on the walls and ceiling on the other sides? It seems surprising that there would be such extensive moisture and you've not noticed any smells or anything in any adjoining room? Where are the tiles that need replacing? You can't replace any floor tiles without pulling the whole thing up and redoing the waterproofing layer. I'm not sure how far up the wall you need to go these days. They used to do just one tile height and around the tap protrusions but these days it seems more typical to waterproof the whole shower to the top of the tiling. As the walls are not plasterboard but fibre cement sheeting I'm not sure why that is. Any additional precaurions are welcome I suppose but I wish they'd get the floor right. Rarely heard of problems when they used copper trays, now they paint smelly stuff everywhere and have to come back and redo it. Can you tell it's one of my pet peeves.