1) That is sweet!
2)A few ideas:
-hotsprings as 'points of light'
-abandoned magma-tech stuff
-loss of literacy as books and scrolls were too heavy to carry and were left behind as the races fled towards the equator (or wherever is warmest)
-if you wanted the plane to power magic via leylines, you could have fire magic use lower caster levels or not have as high spells to reflect the loss of so much fire magic (ie. if this was E6 which caps at 3rd level spells, fire magic never gets past 2nd)
-cold magic could get a boost or many fire spells get reflavored- flaming sphere becomes chilling sphere, fireball is cryoball or something better
-digging to where is is warmer, the dwarves or whatever "miner race" may have found geothermal energy and dug deep to stay warmer.
-ash storms hinder breathing, visibility, flight of animals, and even movement on the ground with winds and increasing amount of ash on the ground
-if it stays cold enough for enough time you have sea levels drop, glaciers form, woolly mammoths (what party doesn't want to hunt one and have "woolly mammoth hide armor"