I see no reason to think that there were "several meters of snow" over everything. Canonical descriptions speak of vast tundras where the Doviello roamed.
Technically, Erebus is a flat, infinite plane, not a globe. There is no reason that north or south need to be colder than other directions.
If the Age of Ice is Erebus was anything like Ice Ages on Earth (or even like the clime of Antarctica), then heavy precipitation (including snowfall) would have been rare. It is not as clear in Erebus as on Earth that precipitation requires prior evaporation of water though.
Ice doesn't mean literal frozen water, but Cold. In Erebus, the sphere of Cold is the manifestation of Nostalgia. As such, I imagine that ruins would have been left visible enough to serve as reminders of better times gone by although ruined enough to be of no practical value. Mulcarn would have wanted humans to idolize the past, to yearn to go back to how things used to be but not to have hope of returning to or surpassing that glory.
The technological regression was likely more severe than that of the so called Dark Ages in the Middle Ages, but for most civs probably not too much worse than that of the Greek Dark Ages (which included the loss of the concept of written language). Some civs regressed far more than others, and some did not have as far to fall anyway.
The Amurites had no real connection to Patria, and were quite primitive when Kylorin found them. He led them to be more advanced than most, but not by that much.
The Elohim preserved the knowledge of the Age of Magic best. Some of the greatest libraries of the previous age remained intact until the next, and the people remained mostly literate. They lost control of most of the sites Sucellus had entrusted them to guard, but the hardships of the age may actually have helped them get back to their founding principles of caring for those in need.
The Luchuirp were probably the most advanced civ in the Age of Magic, but were hard hit by the climate change and regressed greatly. They had already lost the skills needed to live underground and had to relearn them as humans and elves did. The art of crafting golems was not completely lost, but it regressed to the point that few could make them to be more than toys.
The Age of Ice did not cause the Khazad to regress at all. However they were not the most advanced before that, as they had isolated themselves underground for centuries before that and never learned of the advanced made on the surface.
The Ljosalfar and Svartalfar were hard hit by the cold, but there are still plenty of elves who were alive during the Age of Magic (and some from the Age of Dragons) whose skills were not hurt much by a few centuries of disuse. Additionally, Cernunnos himself dwelt with the Ljosalfar during the Ice Age and may have led them to progress instead of decline or stagnate. The Winter Court was previously more advanced than the Summer though, and probably better adapted to survival in the cold so they aren't now far behind.
The Lanun survived the Age with ship building and sailing skills intact. (The Lanun existed before Patria and were never fully integrated into the empire; nominally they were all forced into vassalage, but they lived too far from the Patrian capital for Kylorin's authority to actually mean much. At the height of the empire's power they frequently granted refuge to enemies of the state, and overcharged the imperial armies for the service of tracking down their fugitives.) Many waterways became unnavigable, but the Aegean sea remained warm enough for maritime life to go on. The fact that the sea had become wild instead of tame when Danalin fell asleep meant the Lanun likely had to make technological advancements in order to keep their ships afloat.
The Illians were the least advanced civ in the Age of Magic. They did not regress, but neither were they allowed to progress.
The Bannor who stayed in Erebus regressed and became Orcs, even though their homeland stayed fairly warm. The Bannor to spent the age in hell did not really regress or progress, although their numbers were too few to build as advanced a society when they emerged. Also, the fact that these were mostly members of the nobility and priesthood from inner city of Braduk means they likely could not preserve the technical skills of the lower classes. Most Bannor of the current age are descended from scattered tribes who allied or were conquered by the real Bannor.
The Hippus were never that advanced. (These too existed before Patria and were more like vassals than full members, but these vassals provided a large part of the empire's professional army, especially the cavalry.) They always depended on their livestock, not only horses but also sheep, cows, and goats. Some of their finest breeds died out or went completely feral, but they were able to save some of their flocks and the basic skills of animal husbandry. (The Hippus were however never one people, but a diverse group of clans with similar lifestyles. I'd be surprised if the modern clans actually be traced back very far, but there have always been some such clans in their steppes since shortly after the signing of the Compact.)
The Calabim of the Age of Magic have no real connection to the modern Calabim except for sharing Alexis and Flauros as leaders. I like to think that these two have on more than one occasion slaughtered all their followers in order to keep the source of their power a secret. The current Calabim are descended from a tribe no more advanced than the Doviello, who made a pact with the vampires when it seemed there was no way to survive without their superhuman strength and senses to help hunt Mammoth. The leaders however likely remember the Age of Magic and even much of the Age of Dragons quite well and can make their civ advanced quite quickly if they so desire. Flauros, at least before the rise of Decius, however seemed to think that staying small and seeming primitive on the surface was a better defense that projecting strength.
I'm still unclear as to whether Perpentach was broken out of the Palus during the Age of magic, Age of ice, or Age of Rebirth. I prefer the last of these, Nikis-knight prefers the former. In my scenario: the Age of Magic Balseraph empire was what remained of Patrian after Kylorin abandoned it and led other regions to rebel. Laroth, Gatrius, et alii may have been important leaders here, but as Kylorin's adopted son Henri Ghouls would have been seen as the heir apparent and would take the throne if it were not given to him freely. He was likely the strongest of them all, and could dominate the minds of any and all who opposed him. He may well have eventually dominated the minds even of his most loyal subjects, simply to make the empire run more efficiently. He was before his mental barriers where broken a very cruel, somber, calculating man who did not understand the concept of mercy. The old Balseraph empire would have been a gloomy place where no one would dare tell a joke. It was only after centuries of insanity that Perpantach came to enjoy his madness. It may also be that he could only move his mind, and not his soul, into his new hosts, making The Momus a much more jolly character than the emperor of old. In Nikis-Knight's scenario Perpantach was not imprisoned for very long, but escaped in time to found his carnival kingdom before the Fire Rain and lead it though the Age of Ice intact. Either way, Perpentach's mind contains the greatest minds in history whom he could call upon to recreate nearly any advance of prior ages if he so desires. I imagine his copy of Gatrius is an important adviser; the Master of Metamagic is almost by definition has the greatest memory in Erebus and knows far more obscure lore even than I do. Gastrius also seems to be perhaps the most cooperative mind within a mind. Of course, little Henri may not have the will to listen to him and at times would prefer more entertaining but far less useful advisers. The Balseraphs are likely not the most advanced civ simply because Perpentach doesn't care enough about advancement.
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I guess it's interesting to wonder how much of any past civ's culture has made it through the AoI, and how it survived, which depends on how severe the AoI was. I suggest this is more of a topic for the lore subforum.
