Actually, if Earth were flat, it would be up on its side, perpendicular to the sun, and centrifugal force would provide the gravity necessary (rather than internal rotational gravity). All that would change is the length of the year and we would have sun all the time, except when other planetary discs block the sun from us. The moon would still orbit around us on the edge of the rim, dipping up and down on the horizon.
As for the magnetic poles, it would still have them, just like a magnet of any shape still has positive and negative poles (and the protective magnetic waves). The planet would have adapted to the environment and formed appropriately to support life.
Hypothetically, anyway.