Given the amount of material you've set out, I think it'll be easier if I just give you feedback in point form.

First off, I quite like the notion of the sentient Prime races teaming up to beat up the whole Pantheon - 'good' and 'evil'-aligned clerics and paladins (or blackguards) would be a the core of the world's forces as their magic and powers are particularly effective in permanently taking out demonic or angelic entities.

Second, you might consider coming up with some shorthand for your calendar, i.e. some way for more common folk to keep track of the days and months as your formal system produces rather lengthy and complicated dates.

Third, how would the diplomat status of the head of the Steamforged Knights play out given that they are based in Terrantis and that the status only applies there. Does it provide the head with some sort of immunity from prosecution? Is the property of the Steamforged Knights not subject to Terrantis' laws?

Fourth, I love to hear more about the unknown entity that murders anyone entering Umbarius, though keep some of the vagueness. Particularly, how precisely did the wizards figure out to protect against it? Camouflage or a more straightforward shield? Could the protection be related to something particular to that entity?

Seventh, please expand on Necros - it sounds really, really cool and the consequences that this all would have for a society are mind-boggling. What position would more sentient undead have and is there some way for ordinary undead to ascend after their term of post-humous servitude has run out? Are zombies perhaps modified to make them more suitable for their particular tasks, e.g. an implanted lantern in a zombie's chest for guides or added muscles to make them better dockworkers?

Eighth, where does water and food come from? Do they siphon water up from lakes or collect rainwater? If its the latter, the islands would be particularly disliked by those stuck on the ground as a lack of rain could lead to draughts, in addition with the large shadow the island itself would cast over the land. Are there special farm islands? What about animals and other needed resources like wood or stone?