Sorry for the delay in getting back to you on this. The Z setting is a bit tricky and can be confusing so lets see if I can try to explain a little clearer.
The main problem is that the Corian is typically 1/2" thick and either needs to be machined to around 1/4" thick or an initial 1/4" deep pocket needs to be machined. The Lithophane is then cut into the pocket region.
When using the pocketing method, then you are correct about 'cheating the Z zero position'. This is because the Z0 position for the initial pocket is on the surface of the material - then run this toolpath.
In PhotoVCarve the 3D Lithophane toolpath is also calculated relative to the material surface. So you need to reset the Z0 on the machine to now be off the bottom of the machined pocket.
You should be able to do this with the XY Origin in the middle of the job by,
Cutting the pocket
Change the cutter
Return to the X0 Y0 point in the middle of the job
Drop the cutter to just touch the base of the pocket
Set this = Z0
Run the Lithophane toolpath
Very Important - In PhotoVCarve make sure the Rapid Clearance Gap (Safe Z height) is set to at least 0.280" so the cutter can move across the top of the material.
Note - When you get confident in using PhotoVCarve and VCarve Pro it is possible to setup the complete job in VCPro and not have to reset the Z0 origin off the bottom of the pocket.
Import the PVC file into VCPro
Double click / Edit the toopath
Edit the Start Depth to be 0.25"
This drops the whole toolpath into material by 0.250"
Save the Toolpaths using VCPro
If you calculate the Pocket toolpath 0.25" deep in VCPro then Edit the PhotoVCarve toolpath to have a Start Depth of 0.25". Then Preview the pocketing toolpath then the 3D lithophane toolpath. You will see that the lithophane detail is cut into the base of the pocket.
I hope this makes sense and help?