You're mixing up laws with models.

Conservation of momentum is a physical law, that is, we're not making that up, we always see it. The Navier-Stokes equation is a continuum mechanics model of fluids with interesting mathematics, but it's not fundamental, it relies on conservation of momentum and additional reasonable assumptions about the fluid, in the same way that for instance the van der Waals equation is reasonable but it's not a law, you find deviations and there are better models.

As far as we can tell the laws of thermodynamics are laws indeed, that's why no one is proposing perpetual motion machines of the second kind any more.

The law I was referring to in this case was Stefan–Boltzmann law (heavily reliant upon assumptions on a given black body that break down under certain circumstances, that people like to hand wave away because they haven't had to deal with the complexities of said circumstances before), which may as well as be a model as far as I'm concerned.

No, it isn't, it's what you get after integrating to all frequencies the Planck distribution for the radiation of a black body in thermodynamic equilibrium, which if you can get a sufficiently black one, you can check out it matches perfectly. I mean perfectly, that's what remote sensing satellites start with for determining surface temperature, after convenient linearization and modeling emissivity. That's what we see in the CMB, again perfectly. It's like this, I'm sorry.

(Look at the CMB spectrum —and refs therein—: https://commons.m.wikimedia.org/wiki/File:Cmbr.svg)

And then take those same sensors and apply that to the said materials I mentioned above, above and it breaks down…

"Oh but we haven't observed it in the cosmos from our cherry picked backtested 'sufficiently black' ones so it doesn't matter"

Rah rah rah, if you want to continue more just, email me cinque [underscore] mcfarlane - blake [ - a - t] brown [- d - o -t] edu