The drawing doesn't show the coping stones crossing the boundary, so it's unclear what the issue is.

I suppose a rail may accommodate more coats, but will you get that many in a cupboard of that size? Here’s some ideas, but do have a look on Pinterest there are loads of images.

It's really hard to advise you without knowing your lifestyle, food tastes, food habits and your cleaning habits. Have you looked at reviews of the actual sink from human users? Blanco are trying to sell you one, they are not going to point out the negatives. I signed a disclaimer on my cararra marble worktop for my island when I had it installed. We are careful and 6 years later when we sold the property it was still perfect. Think about whether you are prepared to invest the extra time and cleaning effort to keep it pristine. Do you have another sink for prep? Read this first before committing. https://www.houzz.com/discussions/4107932/reviews-on-white-silgranit-sink

Thank you. For further context, the garden area behind the current living room (to the top of the floor plan) is where we get all the afternoon and evening sun, with no neighbours to the right hand side either, so we don’t want to build out there if we can help it. Although would consider a small extension if it made sense internally. We do have around 2 metres width of garden front to back alongside the living room and w/c wall which is currently space we don’t utilise and could be used to extend (the fireplace flue is external and could be removed).