I, too, believe Propast to be real. Like several others have said, Zoe just "dreamed herself" there, in the same way she dreamed herself into Arcadia (in DF and then again in DFC). She disappeared from both in the same way as well.
There are actually story-related reasons for Propast being real. Had there been only personal developments for Zoe (and maybe those closest to her) in Propast sections, this theory would have been plausible, but, IMHO, there's been too many plot elements important in the bigger picture (hell, the political situation in the whole region is at stake) I really don't think all that will have had been a dream. It would not add up narratively.
As for the discussion. I think Propast is real. Zoe was dreaming in Casablanca, and dreamed herself to Propast, just as she has to other placed like Winter, Storytime, and Arcadia. So all the events happened in the real world, only Zoe was doing them with her "dream body".
BTW (slightly off-topic), was Faith's limbo ever called Winter anywhere ingame, back in DF? Or was it just in the promotional materials? I replayed DF recently and I don't remember anyone referring to it as "Winter".