Perhaps the most convincing way to find out how X-Plane mesh works, is just test it for your self. I did.
But it is always good to check other sources as well.
Mesh overwrites everything below
These sources explain that the base mesh overwrites anything below it:
Only 1 mesh file will be loaded per tile
These sources specifically explain that only 1 mesh will be loaded:
If you would like to know more about the X-Plane file formats and DSF files in general please check these links: