Thanks for engaging so thoughtfully. I'm enjoying reading and thinking carefully about what you wrote.

Is the past unchangeable once it has happened? I suppose you're correct that I assumed this to be so. I suppose you're also correct to imply that it's an unprovable assumption—as I think about it, it doesn't seem like I can really prove either case (mutable past vs immutable past). But, it seems to me, assuming the past to be immutable is safe for me to do because I will never be "bitten" by that assumption. In other words, even if my assumption "turns out" to be wrong (whatever "turns out" might consist of), I risk nothing by acting as though it's right.

Here's some thought experiments supporting my stance:

1) Suppose that the past can be changed, and that change is guaranteed to have complete and consistent consequences on the present—that is, the flow of time, once changed, will be as though it had been that way all along. In that case, from my perspective in the present, the "updated" present would "always have been the case" and the change would not be detectable. But, I might argue, that being the case, there's really no downside to me operating as though the past cannot be changed. Any conclusions I reach based on that assumption will be internally consistent with the rest of my reasoning and experience of the present, even if the assumption is untrue from a perspective that I cannot observe the timeline from.

2) Or, suppose that the past can be changed, and that change is guaranteed to NOT have consequences on the present. In that case, from the perspective of the present, every indicator that I have of what the past was could be exactly the same (even my memory of it) and yet the past as it "truly" is has been altered, making all of my indicators unreliable. But, I might argue again, since from my perspective the past changing without consequences on the present is indistinguishable from the past never changing, I'm again safe to assume an unchanging past without any difficulties being introduced to my reasoning or experience of the present.

3) Finally, suppose the past can be changed, and that change is NOT guaranteed to have complete and consistent consequences on the present—that is, some detectable artifact of the past being altered could be created in the present. In that case, I will go ahead and make a bald assertion: No artifacts of alteration of the past have ever been identified or will ever be identified. This is a plainly falsifiable assertion. I cannot continue to hold it if it is falsified by evidence to the contrary. But until that point, I'm comfortable accepting it as valid, since it agrees with conventional reasoning about time and, as far as I can tell, is consistent with my experience and the rest of my system of reasoning. In other words, I see no compelling reason not to operate from that assumption as true, even if it is unproven. My system of reasoning has many such positions in it.

So the burden is now on you to prove that the past has indeed changed and show me how you know that, otherwise I'm content to continue assuming as I have that the past cannot be changed (even though I admit, as you've forced me to, that this assumption does not rest on evidence-based proof but on lack of proof otherwise).

So let me now consider your argument that the past is, as you say, "quite mutable." Honestly I'm not sure you've proven that the past IS mutable, just that it COULD BE mutable. And you've told me that you think it is. That isn't compelling enough for me to eliminate my standing assertion(s) above (even though I grant the substance of your challenge).

It occurs to me that there's another angle I could pursue on this, which is to examine what we mean by "time" itself... But I that might be a little beyond my abilities to reason out, so I'd better stop here.