the counter was nice and shiny but now it has become cloudy and doesnt look good at all. how can i fix this

Any pale colour will stain if you treat it badly so even quartz which is considered to be hardwearing will need care so you don’t leave anything on it that has a strong colour (such as red wine spills etc). Beware- the inexpensive quartz seems to be more plain while the more veined patterns can be three times the price. If you are comfortable with a higher price range you might consider porcelain style products like Dekton where marble like products are widely available.

The cardboard backing is a balancer. It’s a necessary requirement to keep the worktop level to prevent wharping. Out of interest was it from Howdens as they’re well known for cardboard backing while other major suppliers offer a different material which is better.

It is probably better to keep to one height in the sink corner, for all the above reasons. This time round I'm going free-standing. I discovered one height all round affects my shoulders - the worktops are too high for my height and different jobs (eg mixing, bread making, carving, chopping) and it seems there's a standard height just to fit the appliances underneath. My current, old, about-to-be-redone kitchen served its purpose but practically didn't work so well. So this time I'm including working heights that suit me, not the designer \ trader.