iirc, none of the celestials show up in anything remotely Zilart, just the terrestrials, right? It could be they honestly didn't know of their existence, or the proto crystals weren't there then. The closest we see anything remotely Zilart to a cloister is the headstone in Terrigan, right? Wait, I forgot that ice is in the basement of Fei'yen. Hmmm. The honestly could have done nothing to them, maybe the avatars weren't in the somewhat semi-awake state they are now. The CS I was talking about definitely shows a very dark proto crystal in Bearclaw, though. Perhaps the proto crystals weren't around before Gordeus blew up? The celestials have been asleep since before the Zilart though, feels like I'm missing a key CS or SE just hasn't explained something..
Ugly range is also a bit more north than the glacier is, they could have just not gone that high, it also wasn't an icy wasteland until the meltdown, it could have knocked his proto crystal out of place, waking him up in the process, he goes after Alexander, Gordeus blows up, he's weakened, goes back to the Crystal to recover? Feels fine around the time Raogrimm goes crazy, gives him power in exchange for release?