Vincentius said:
Full spacetime symmetry by the perfect cosmological principle implies a universe in stationary state
Even if we accept that this is true for the sake of argument, it does not mean that the perfect cosmological principle is the
only possible way that, for example, the fine structure constant could be constant, not changing with time.
Vincentius said:
The standard cosmological principle does not provide time translation symmetry
Indeed not, since our actual observations of our actual universe on cosmological scales do not show such symmetry. So of course our standard model of the universe does not have it either.
Vincentius said:
so does not guarantee general conservation of (dimensionfull) physical quantities
Dimensionful quantities are the wrong ones to focus on, since they depend on your choice of units. The quantities to focus on are the dimensionless ones, like the fine structure constant. Our standard cosmological model does not "guarantee" that such quantities are constant, but we don't need it to; we can test for that directly by observations, as I said before; and so far we have not found any variation in such quantities.
Vincentius said:
Is there any observational evidence of conservation of mass in cosmology?
No, and we would not expect any, because mass is not a conserved quantity.
There
is abundant evidence for local conservation of
stress-energy, i.e., that the covariant divergence of the stress-energy tensor is zero, as GR predicts it should be.