No problem on the slightly late reply, you must be at least somewhat busy ^^.
So, I understand the 2D grid now as the as the combination of temperature and density producing certain levels of emmisivity (the z-axis), and the grid needs to be interpolated at its missing points because not every combination of the two variables have been experimentally tested on Earth yet. However, now I am a bit confused again. So the standard practice is to use simple interpolation to fill in the grid, and I can imagine that is a bit naive, but how does Bayesian sampling overcome a naive interpolating process in this case?
I am reading your paper, and I sincerely will say it reads clearly and it is organized well, but it is still a bit out of my depth 
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Exoplanet is a well-made package, I hope it works for you as well.