Okay, we are going for the Pliolite for the whole wall (the final
colour, magnolia).

Good. You got your way then!

One question: the painter doing the high bits says two coats are all
that is needed. I suspect three would be better.

I'm with the painter

The first coat should be diluted with 10% white spirit to stabilise
the surface, that much I am sure.

Are you? The solvent for this paint is naptha, so white spirit might
well screw things up. I'd use the stabilising solution recommended by
Johnstones if you think the surface needs it. You're paying enough for
the paint, might as well follow their advice :-)

But two coats or three? If three
will add to the durability and lifetime, even just by a few years,
then that is what I'll do. If a third coat is a waste of money, then
I'll skip it.

Any opinions?

-- Jason