I'm going to try to play an MP game with the Frozen Civ if possible. I was very satisfied with my SP game. I got to Summon the Frozen Civ, the Wintering Spell worked correctly, and I got to make the Ascension Ritual and have Taranis actually Ascend. I was very satisfied.
1. I like the Evil Ice Elf Druid Unit a lot. Regarding the Summon frosted forest ability, or whatever, I wasn't able to cast it because I had already casted wintering and everything was Ice/Blizzard. Perhaps it should be castable under those conditions?
2. My Hawks were suffering Cold Damage from the Blizzards, a couple of other units might have at times (Frozen Souls, and Great People took Blizzard Damage)
3. I like the Swamp of Souls building, it gives me a reason to get the Sanitation tech, I'm not sure about the name however. I don't usually associate swamps with ice
4. Wintering Worldspell was given at Arcane Lore (your post says back to sorcery from Arcane Lore, leading me to believe that the spell would be given at Sorcery)
5. I can build mana nodes in blizzards, I'm very happy
6. Gifting bug from Hyborem is still present. Gifted cities don't have Frozen Citizens
7. Barbarians are still my friends and won't attack me, but I can attack them
8. I'm feeling pretty good. I can go Recon Route to Druids and then Magic route, or Magic Route and then Recon Route to Druids. I don't feel gimped either way and this increases joy.
9. I know it's a temporary measure, but Liches look the same as Archmages (not that big of a deal, but still, and please don't block Liches)
10. Asended Unit is kind of Anticlimactic. He gets hurt real easily, and while my other units have march and heal every turn, Asended has to wait a few turns. Perhaps letting the Avatar get promotions would be good.
11. My Winterborn units (as frozen civ) can still get Wintered Promotion that replaces winterborn (not sure if this is intentional).