I would not like argue about the chronology of the book of Job, because historians do not really know. Historians do not know if it was a post-Moses or pre-Moses book. The books in the Bible are NOT in chronological order. They were organized in a semi-historical order. Still there are many poetic books that have no historical placement so they are just included randomly in the Old Testament.
This is what I do know of Job:

1. The book of Job is very similar to other "Ancient Near Eastern" texts around the Sumerian era. There are MANY books that play out a similar story like Job's; where a deity forsakes his servant and allows bad things to happen. The book of Job is probably a similar adaptation to the cultural stories of that time. So the story of Job was a very common story in its ancient near eastern context.
   Note: there are also many "creation" stories similar to the Bible's one.

2. I like to think that the Bible is a book of ideas passed on to preserve a culture. The book of Job seeks to ask an age old question: why do goob things happen to bad people? why do bad things happen to good people?
   I would not worry too much about if the devil can or not go to the presence of God. Since it is a book of ideas, I think they are just trying to illustrate a point.