One approach that might work rather than simulating physics backwards (which strikes me as implausible given the nature of quantum mechanical effects) would be to analyze the information coming back from interstellar dust particles. This would tend to be impacted only by sources of light and gravity at a given precise distance.
Thus a crude model of the large planetary bodies of the solar system at a given moment could be calculated by triangulating information from the correct set of dust particles, which could be refined over time, as new data points are added, to indicate large land masses, and eventually resolved into human bodies and their individual atoms. This should be sufficient to resurrect everyone from their moment of death, as well as restoring every memory they ever forgot.
I don't see this as a good substitute for cryonics because there are plausible universes where FAI is e.g. self limiting in a way that would prevent it from converting the solar system (perhaps the galaxy) into computronium, which I think would probably be a minimal prerequisite to actually pulling something like this off. I also think an intelligence explosion scenario is not inevitable (may be impossible or extraordinarily unlikely for some reason), and that cryonics can work with incremental human-level improvements in technology.
Even in the high-speed fooming FAI class of scenario, such tasks as converting the galaxy to computronium, and collecting enough data from enough points, are limited by the speed of light factor and might take hundreds of thousands of years, if not millions. In the mean time, those who are cryopreserved could be up and walking around, forming new experiences and laying the groundwork for future civilization. They would also be more likely to have opportunities to take part in early interstellar and intergalactic colonization efforts.
If someone gets cremated or buried long enough for eir brain to fully decompose into dirt, it becomes extremely difficult to revive em. Nothing short of a vastly superhuman intelligence would have a chance of doing it. I suspect that it would be possible for a superintelligence to do it, but unless there's a more efficient way to do it, it would require recomputing the Earth's history from the time the AGI is activated back to the death of the last person it intends to save. Not only does this require immense computational resources that could be used to the benefit of people who are still alive, it also requires simulating people experiencing pain (backwards). On the other hand, this saves people's lives. Does anyone have any compelling arguments on why an FAI would or would not recreate me if I die, decompose, and then the singularity occurs a long time after my death?
Why do I want to know? Well, aside from the question being interesting in its own right, it is an important factor in deciding whether or not cryonics is worth-while.