I don't quite understand why AIXI would protect its memory from modification, or at least not why it would reason as you describe (though I concede that I'm quite likely to be missing something).

In what sense can AIXI perceive a memory corruption as corresponding to the universe "changing"? For example, if I change the agent's memory so that one Grue never existed, it seems like it perceives no change: starting in the next step it believes that the universe has always been this way, and will restrict its attention to world models in which the universe has always been this way (not in which the memory alteration is causally connected to the destruction of a Grue, or indeed in which the memory alteration has any effect at all on the rest of the world).

It seems like your discussion presumes that a memory modification at time T somehow effects your memories of times near T, so that you can somehow causally associate the inconsistencies in your world model with the memory alteration itself.

I suppose you might hope for AIXI to learn a mapping between memory locations and sense perceptions, so that it can learn that the modification of memory location X leads causally to flipping the value of its input at time T (say). But this is not a model which AIXI can even support without learning to predict its own behavior (which I have just argued leads to nonsensical behavior), because the modification of the memory location occurs after time T, so generally depends on the AI's actions after time T, so is not allowed to have an effect on perceptions at time T.