[continuation of too long comment]

"He is essentially arguing for a fixed background of indivisible non-metric time that everyone experiences universally and that unrolls at a fixed rate for all observers."

Right, that pre-Einsteinian picture has proven to be wrong. Accurate clocks at different altitudes and moving at different groundspeeds bear this out, even if people living on mountaintops or flying in jets don't notice the parts per billion difference in their day from the people living at sea level. The GPS tools they have with them do, though.

And, sadly, he did not live long enough to see 1971 ( https://en.wikipedia.org/wiki/Hafele%E2%80%93Keating_experim... ).

Penultimately, there are some theoretical physicists who think time is "real" in the sense that it is fundamental rather than just emergent. I think you are taking an emergentist position (which I agree with) when treating it as arising from observed periodicity. (Remember that your observation of something's period -- like the bouncing light pulse between the parallel mirrors -- is not necessarily the same as another person's observation of the same something.)

Finally, just to bend your brain a bit, in General Relativity in any universe which is even close to being like ours, you cannot have a system where a pair of mirrors with a light pulse bouncing between them can be forever parallel. The parallel mirrors and light pulse are a system of mass-energy that source very slight (but nonzero) curvature. That curvature means that the parallel mirrors, if close to one another, are on a converging path even in empty space far from all other matter. If far from one another, the metric expansion of space means that the parallel mirrors are on diverging path. In a completely empty universe with a finely tuned dark energy, one can set up a classical system in which the system is extremely finely balanced so that the mirrors will stay the same distance apart (measured locally by a notional mass-energy-less observer moving with the mirrors), but real mirrors and light, made out of parts of the Standard Model, will break that fine balance, and the mirrors will move onto either a converging or a diverging path eventually (maybe bet on diverging because of the relative strength of the electromagnetic interactions with the light pulse compared to the gravitational potential energy, and because real mirrors are imperfect reflectors so some photons will "leak away").

On top of that, a really long (approximately "straight-line") Twin Paradox journey in an expanding universe can put a cosmological horizon between the Twins, so they'll never be able to compare their wristwatches in person. Each will see the other slow down and grow dimmer, but only the one moving at near the speed of light (still locally constant everywhere) will live to see her twin disappear completely across the horizon.

(Of course a similar journey confined to the neighbourhood of the Milky Way, e.g., by zipping to and fro many times, will not involve a cosmological horizon.)

"post-Einsteinian space-time"

Well, we call it post-Newtonian. General Relativity's fundamental theory (and in particular the Einstein Field Equations) is very much Einsteinian still. We just understand it better than he did, mainly because we have newer calculational tools (and newer mathematical innovations), and because we have the advantage of access to many thousands of relativists' work over the sixty years or so since his death.

> Thanks again for the thoughts, it was a great read with my morning coffee!

Likewise.