IMO, it is fine to call 'c' an absolute but with the caveat that it is applicable to one's own inertial frame. It is so absolute, in fact, that we no longer have a reference stick to define a meter. The length of one meter is based on 'c', because it is an absolute value for a "stationary" reference frame.
"Caveat": Large Hadron Collider (closed system) (closing system).
Opposed: Infinity (open system) (opening system).
Universe.
Multiverse.
How broad is the ocean of the universes?
How deep is the ocean of the universes?
You can't coordinate the breadth. It's too broad.
You can't coordinate the depth. It's too deep.
You might think you can coordinate your own small bubble of it, relatively speaking that is, but not really.
Not trying to be derogatory or facetious, but give me the "stationary reference frame" for the principle of uncertainty writ large. One in which you the observer cannot tell precisely a point of position. Or, one in which you the observer cannot tell precisely a plane of velocity or momentum. (** you are absolutely right for finite-foreground "local", but I wanted to insert the opposing qualification for infinite-background "non-local". After all, regardless of the closed-minded-only who insist that only "closed system" exists, that there is no such thing as an existence of "open system", for one to exist the other must exist. And if one is subordinate to the other, then there are two opposing subordinates in two opposing realms. **).
You're "absolute value for a 'stationary' reference frame" is [coordinately] correct within any one of an infinity of four-dimensional boxes. You would have a big problem between the boxes though. That "absolute value" then loses all its meaning. The speed of light constant will measure its constant within them all, from all points of each and every box in (closed systemically), but loses all meaning between boxes (open systemically), For example: Is that distant object from you, observed to be that distance from you, in fact that distance from you? Is it even there? Is it even that object? Was it ever there where you observe it? And was it ever that object you observe it to be? Did it ever have the "absolute values" you (you in general, that is) observe it to have and give it? Though, once again, "the length of one meter is based on 'c'", is based on a unity of physics, and would be one meter in length within any of an infinity of four-dimensional boxes.
There have been representative illustrations of entire scenes closing up to a line, a bubble contracting into extremities of contraction, then to a point, then disappearing inside some other scene. A universe, still existing in own right locally, having disappeared -- to all intents and purposes, leaving the other "local" universe in its place. Two universes occupying the same space at the same time, ** and/or ** but not the same space and not at the same time. Quantum weirdness? UFO-ology? Breakdown of relativity? The ocean seas of space are mighty, mighty, deep in ocean seas as well as mighty, mighty, broad in them.