I think Layout should be LayoutT to avoid thinking it is the layout itself (as in the content of a UBO/SSBO).
I don't understand this value. It looks like it was copied from gb->size_vertex. For point size it make sense because there was a square dot to circle dot surface conversion (from the 2.8 days).
This should be move right after TH_VERTEX_SIZE.
I think it is nice to link this comment blender/blender#104921 (comment) to this PR. It has some nice related explanation and it prevents to see it…
Yes, but here it is the reference to a specific formula. I also think this reference should be inside the function. Comment on top of function is only for documentation of the function usage.
Maybe also add a note that offset is in viewspace. So positive values are closer to camera.
Remove the negative sign here and everywhere get_homogenous_z_offset is called.
Replace description by Offset used to draw the edit mesh in front of other geometry.
Now that we fixed the bias equation, the property types should be PROP_DISTANCE.