Found a quest that reveals a bit about Xal'atath's plot that I haven't seen mentioned so far.
There's a few quests where Brann asks you to do a particular Delve to investigate Zek'vir. The last one I found reveals that Xal'atath gave him a special mission to erase the Worldsoul's memories. Definitely not the whole picture just yet, but this seems like an important piece of the puzzle. Erasing Azeroth's memories to trick her, or to weaken her resolve to resist corruption? Removing some Titan indoctrination she got from the Pantheon?