Re: c is calculable
"It's widely understood that relativity claims that c is a constant"
Not just Relativity. It comes out as a constant in Maxwell's equations, for a vacuum. A medium allows for slower propagation of light, radio, magnetism, electric fields. A coaxial cable slows radio waves, like fibres slow light. Or PCB traces on a multilayer board slow the signals.
Einstein was understandably pleased when he studied Maxwell's work.