Either way, the symmetry of having a beginning and an end is nice from a human storytelling perspective, but I don't see any reason the Universe should follow rules of plot.
Well, physics shouldn't care about the direction of time. And it's not just because it's a nice story. It's consistent with the argument that the hypothesised beginning in indistinguishable from the hypothesised end. And if they're indistinguishable, then we should expect them to behave the same. But you are of course right, I'm making assumptions, about there being a beginning, and about the final state of the universe being pure radiation. But I think the least unrealistic assumption I'm making is that space and time do not exist in a universe full of photons. That kinda seems inevitable.

I have real difficulties with the simultaneity, here. You're invoking GR to support this hypothesis, but you're ignoring GR in the conclusion.
There are only photons left, the concept of simultaneity kinda becomes redundant. From the FoR of each photon, everything in the universe happens in the exact same instant. You're invoking time in a universe where there is nothing left to experience time. You're an imaginary observer with an internal clock, in a universe with no clocks.

A photon already breaks simultaneity, if this is what you think is happening. A photon that leaves the sun and is then absorbed on Earth by leaf, from our FoR that took just over 8 minutes. From its FoR, it took zero time. So observers on Earth already disagree with the photon about the order of events. The photons says everything that happened during its existence happened at precisely the same time. The observer says no, this happened and then this happened. Simultaneity only holds if you actually have clocks.

I only know for certain that Einstein said so.
Fair enough. In a universe with only photons though, with there only being one FoR that exists, is it still non-inertial? Once we have only photons, this FoR is not accelerating relative to every other FoR that exists... it is now truly inertial, surely? There is no other frame it has relative motion to.

The Universe doesn't seem to have any issues at all with things moving ever-toward infinite values.
Unless the universe is infinite, then decay must eventually be completed. Why would the last interaction never happen? Why would the last particle not decay into a photon if all the others did? Even if the time between each particle decay was increasing at an exponential rate as the mass in the universe slowly evaporated, it's still finite.

If the universe is infinite, then I can see how decay will take forever. But if it's finite, then I can't.