Dear Colin
you are completely right emphasizing the Nernst ideas. Hubble approved it too. The farther he looked in the 1930ies into space the more convinced he was that expansion is an impossible explanation for the vast redshifts he measured. With no expansion BB is out.
The logically following question of the Clausius law of ever-increasing entropy has to be solved additionally. Are we sure that it is applicable to an infinite cosmos?
The dispersion measurement (DM) by millisecond pulsars give a chance of determining the redshift mechanism.
Once we have found an experimentally proven cosmological redshift law (Refraction, Diffraction, Dispersion, etc) we can think about what happens after another trillion years.
Best to you
Lutz