Waterfall material same as cabinets rather than benchtop
You'll notice CEK actually calls it a laminated panel, not a waterfall.
The OP called it a waterfall when technically what they meant was a laminated or melamine end panel.
Install marble bench top after handover possible on tiles?
I'd just see if they can leave the benchtop off completely. If they wont, have a very basic 32mm laminate top installed and then remove that after handover and replaced…
Gap between wall and wall cupboards (not suppose to be)
There is always a gap where the cabinets meet the wall. It gets filled with a scribing fillet. 30mm is pretty standard.
Carpenter and Cabinet Maker