I'd say it could be retconned if the sole source were Gaius, but with Nabriales referring to him in similar terms, it puts them in a harder position. Personally I think they made an error by offing him in HW and are trying to remedy it now. I think one thing which was weird was this interview that Ishikawa gave - her answer to me was non-committal as to him being truly "gone".
Pandaemonium could touch on any number of things, but if we go from II, with Mateus as Emperor of Heaven and Hell, one thought is the Seven Hells and Heavens, since we've no real notion of what they are barring some very prosaic descriptions in the setting and the lorebook, plus some knowledge that the aetherial realm extends both downwards ("Underworld") and upwards.
As you say, we'll see how they handle it.
