wow, lots of questions! (kidding, i love your reviews)
Up top, upstairs and all other reference like that mean going to one of the many world's in the multiverse, but often refer to ours.
Actually, time passes oddly, and they've only been down in hell for about 4/5 days.
Demon's and mate are kind of like humans and marriage - except that because a demon technically can live forever, they choose MUCH more carefully. There are partnerships which break apart after a hundred years or more and the two demons go happily in opposite directions, but it doesn't happen often. with human and demon mates, whether they get made demon or not - those are not reversible. for example Sitka and Jahke, they are now bound together for life, it's part of the wish contract. A demon doesn't often describe someone as their mate unless they are really sure they want to stay with that person for eternity.