Totally agree with your builder friend, there are so many cornices available, from inexpensive by gyprock suppliers to specialist plaster suppliers.

In my opinion, Shadowline, do nothing to enhance a room,

Some are sold and installed with a tear off strip of plastic, to be removed after installation, this may not have been fully removed,

I think a simple cornice will look much better and go with the lambs tongue skirting. Since this post we’ve discovered none of the walls are straight and square. The worst wall being out 30mm!! Looks like we will need to get that addressed first and might be what is contributing to the shadow line turning out the way it has. Thanks for your opinion :)