You may find the easiest way is to buy 12mm lath as this will bend Around the curve easily. You can double it up to give a good thickness. If it is still too bouncy to nail the tiles, then you can screw them.
With Marley Modern tiles you do not have the half tile.
The half is called an eave tile and is there to weather the joint in the first course of plain tiles or slates.
It is not required with interlocking tiles.
Every slate should have had new nail holes made in them. It is not acceptable to re-use the old huge holes as he has.
There are a few slates on show that should not have been used, due to large top corners being broken or missing bottom corners.
On top of every course there should be a soaker butted against the wall.
You should then have a lead flashing chased into the wall and dressed over the soakers.
I am glad your happy.
I hope you roofer fixed ply between the battens or plenty of extra battens, otherwise the lead flashing will sink between the tile battens.
The work is done to a terrible standard. The tiles below the window should not be kicking up and the tiles at the sides are kicking up way too much and are not even straight.
It is not good practice. If the wall needs to be rendered then a separate flashing should be chased into the wall before the render bead and rendering is done.