Can you take some pictures, both of the vanity top and the ensuite where they are going? As far as I know it is possible for stone to be cut on site (I've seen the service advertised), but make sure you can check the previous work of whoever you get to do it. As for the taps, you need to talk to your plumber - if there is no room at the back of the bowls, the two options you have are offset to the side, or set into the wall.

I have just measured mine. And the spout is 50mm above the top of vessel. Your vessel is 150mm deep, so the spout is 208mm above the bottom of vessel. This is plenty of room for fall of the water. I would recommend a swivel basin mixer, which allows the spout to be swivelled out of the way. This is an advantage when washing your face. No bumping the spout with your eyes closed.

Save big money on importing one from the UK and spend it on design to make the space work for you without the need for such an impractical solution. There are some cute, tiny wall mounted hand basins which would be a neater solution. https://www.reece.com.au/product/basins-c448/wall-mounted-basins-c452/axa-cento-mini-wall-basin-right-hand-with-fixing-1-9505814 If there is no other option here is another type of toilet/handwash product, but I don't think it looks any better than the Caromo design, although overcomes some of the impracticalities mentioned in the previous comments. https://www.reece.com.au/product/basins-c448/wall-mounted-basins-c452/roca-w-w-integrated-washbasin-white-5-star-9505427 Best of luck, Dr Retro of Dr Retro House Calls/Dr Retro Virtual Visits

A vanity bench can be made to any size. You can still have your storage at 1.5-1.6 metres, but have a bench top running the full length of wall as drawn. The builder is able to install anything that you require which is drawn on your plans and in your budget. Why is the designer saying they cannot provide a long vanity? Is this not in your contract monetary allowance? With a bench top of 1.5-1.6 metres, two basins will be squashed, and would leave no room for bench space. You have two options, either only have one basin installed, or once the home is built, put a freestanding drawer unit which is a similar height to your vanity, along the same wall.