Others propose the existence of extra dimensions of time. If this were true, the timeline we are on including our sense of history and future would be like a vector on a plane and not a line per se. Thus a change could be made to our "history" but we would never know it happened because our present and future would be changed as well.
At any rate, whether history is solidified or is malleable, and regardless of which direction the arrow of time is pointing, God is certainly not bound in any respect to our sense of time, much less a timeline. As you say:
Yes, I have read a little about this theory, over the years. It is likely that my little conceit was subconsciously derived from it. I tend to dismiss the "multiple universes" idea, mostly because I cannot really understand it; and it does not fit my limited concept of the nature of God. My opinion, I know, does not change whether the idea is valid, or not.
Thus a change could be made to our "history" but we would never know it happened because our present and future would be changed as well.
Again, this seems wrong, but only because I am unable to understand how it could work. Nevertheless, I both believe and strongly assert: God is perfectly able to change the past, and to do anything else which He chooses to do. (And, as you say, He could already have done so, but we are not aware of it. )
DG