I can't provide any critique on the science part. But as far as atmosphere and general plausibility goes, I'm more than satisfied with the current state of art. It's always nice to have such details set in stone (sort of), it's like getting the "Director's Cut" version of the game, the version that is as close to the author's vision of the game world as possible.
Also, you don't have to make up stuff when the players ask you what the weather is like...
The magnetic poles shifting and causing the rotation of the planet to change is questionable at least according to our current understanding of planetary physics. There are plenty of bodies that have a magnetic field that does not correspond to their rotation.
That said, the unknown nature of the initial cause of the disaster could be your way out of that. By saying that the rotational pole shifting was unexpected and that the best minds have traced it to the field shift and previously unheard of geophysics.
The strongest statement you need to make is that it did happen. Why it happened can be up for theories and speculation.
As far as I understand, such drastic changes rarely occur instantly in nature... it's more like a long process, probably a few million (or more) years. So there has to be some external factor.
Our planet is a water balloon filled with molten iron. If it suddenly started spinning in a drastically different manner that would be the end of it, new asteroid belt.
However the magnetic poles could give out rather quickly. Magnetic compasses would be useless until they reestablished themselves, and cosmic rays would hit a lot more of the surface of the earth. Aurora borealis would be visible from almost anywhere, while the iron inside our planet shifts around trying to figure out where the new magnetic north and south poles should be. This could take years, which is super fast on a geologic scale. It wouldn't be very healthy. People alive during this time would have a higher incidence of cancer.