Leave a 15mm gap between plasterboard and insulation and fix 25mm boxes direct to insulation.
I take it they're 95x45 studs? I can't see there being a great deal of benefit in having insulation hanging out the back of the studs, if there's any water ingress there it'll soak your boards and...
Depends how much of a stickler your electrician is but he wouldn't be able to dead test the circuits unless the relays have a manual override (doesn't look like they do). You could feed the coils with a separate supply but that'd mean your lights go out with the sockets (for example).
R1+R2 = 0.5 and Ze = 0.3?
I don't know where you got this number from. A parallel path reduces the resistance it doesn't increase it, regardless it's negligible.
Not sure your maths is adding up there. To get the prospective fault current you need to know the Ze and R1 R2, the Zs.
To work out the touch voltage at that fault you would need to know R1 and R2 independently e.g. if they were both 0.4 ohms the touch voltage would be 230/2 = 115V. I'm not...