What we record as the past is as fluid and conflicting as what is recorded in the Bibles.
But the narrative is simply the narrative, it does not change the reality of what happened.
If upon my death bed I have a life review and see my dog running through the yard is every blade of grass in the same place or did God's GAN upscale a blank patch labeled "grass" into freshly created grass because the location of the blades of grass doesn't matter?
What you experience is simply a fantasy and will be whatever you create within your personal fantasy.
It is separate from any reality beyond the fact that it is the fantasy you created for yourself at that moment.
If it is not recorded or retrievable somehow - if it is truly gone forever - then the past is not fixed because there are an infinite number of pasts that could have led to this particular present.
But there is only one of those infinity number of pasts that DID lead to this point. That past is fixed. All the possible pasts that did not lead to this present are irrelevant.
Parts of it may never be known but where it is recorded, we can know. And what is recorded often tells us what WAS NOT recorded. The shape of fossil leaves tells us about temperatures, the size of fossil raindrops tells us how hard it was raining, minerals left in ice cores tells us about event that were unwitnessed.
If it is not recorded directly or indirectly then it's not gone forever, it's simply unknown.