3a. For those who have completed the game, after the first fight with Guilford, is there a later way to go back to Demonis and Hades?
Yes (from New Kordar to Hades)
3b. As can be seen from the Demonis map I've attached, more than half of this realm can't be explored. Is this intended (you know, it's not a bug, it's a feature ), will it open at some later stage of the game or is it a, well, bug?
I never managed to enter the area. Think it was cut away, same as Marshan Swamp.
4. In Hades, the flying platform near the entrance from Demonis takes you to a rocky place where there's an inaccessible location called Corrupted Sanctuary (see screenshot). Another feature/bug or will it open later in the game?
Yes, another Not_Implemented_Yet.
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