2. If the holocaust happens and no one remembers it, and no evidence of it remains, did it happen?
In the first case, no person is around to interpret the information. In the second, the mutual information between the past event and the present becomes negligible.
Random thoughts on the second. From an information theoretic perspective, you can show that if something happens far enough into the past, it will have only a negligible effect on the present. The past eventually becomes unresolvable. Given the present state, the "true" past becomes indistinguishable from any number of possible past states (if you go back far enough). And in the end, the distant past, having no discernible influence on the present, ceases to matter. You can also apply this to the future. Go far enough into the future and it becomes unresolvable given the present.
This is simply a restatement of the Second Law of Thermodynamics. Entropy increases over time, and the amount of "useful" energy decreases over time.
If you believe only in the physical universe, eventually, it won't matter that the holocaust happened; just like it doesn't matter that some caveman you've never heard of suffered and died thousands of years ago. For all intents and purposes, the holocaust will not have happened. So why worry?
Another thought. True revelation is an entropy-decreasing event. It violates the Second Law of Thermodynamics, just as Creation violates the First.
