Happy New Year, RLG
I get the impression that your statements and defenses in your post are quite furtive and demanding. Like you are very anxious about certain developments.
It's likely you are making coherent statements from within the box of the relativistic storyline and about the relativistic invariance imagery that you've mastered and that you like to repeat. However, beside being correct from within that box, it looks to me like what you are not allowing for is the natural emergence of additional (slightly more useful) ~equivalent expressions and adaptations.
How I phrase it is to point out that we're experiencing the shift from the flat-to-curvy-to-nested paradigms, or conceptual models and in the nested imagery, the developing truth is experience exists; time does not.
Thus, where you drag forth the linguistic artifact of the so-called 'thin present', granted, if we all mesmerize to the familiar tune all of the must be's and have to's that you demand seems so very logical. Yet, from within the nested fields within nested fields motif you (and Einstein, et. al) apparently don't notice when you dissociate from one of your ingrained internal models to another.
Perhaps it is just our overwhelming enchantment with words, but please notice that before you get to a differentiated 'thin present', you sort of have to first upload and ingrain the entire basic past-present-future storyline, like we ALL typically have done. This storyline COMES with the assumption of a very thin present. But more than that, from within the nested structured~duality or the nested fields within nested fields model 'the past', 'the present' and 'the future' are also just three different structurally coded widgets -- linguistically encoded secondary associations, or perhaps what Serge might call different intellectual products, or with respect to our love affair with 'time', what is a mis-shapened part of our western scientific spiritualism or religion. That is, time, it's a deeply ingrained but still erroneous paradigmatic assumption.
I suggest that the anxiety associated with letting go of that scientific~religious-like belief prompts your agitated demands for the 'true nature of time' a la the flawed or limiting argument in your post.
Again, I suspect you correctly state the partyline position from within the box. But the point is, that does nothing to advance the ongoing shift from flat-to-curvy-t- nested. And notice that even when you get through you have relativistic space-time nested within at least a few other levels of enfolding intellectual products, so where do you hide or how do you express the overall conceptual model of it if not as nested curvature within nested curvature, or more simply as it is as nested fields within nested fields?
I'm suggesting a non-dual approach -- both and more -- and that there is or are more that one more unified, more accurate and useful conceptual models that match up with our overall experience. Curvy is the initial approximation which obviously works well. But the nested ones are the more unified and more robust ones which will serve to carry us forward and further.
It's a matter of changing the questions.