1) Just how useful the FRCG is for a DM starting a
FR game
It will be useful since the world has changed. The Spellplagues have happened and the world moves on.
3) Is the Player's Guide much of a neccessity as far as flavor/fluff text goes (i.e. are there details on regions/peoples/happenings that are not present in the campaign guide? or is it mostly crunch?
More crunch, but what it does do is present the players with info that they would need. Having a player read this book would be fine but havign a player read the campaign guide might not be.
4) I was told that the
4E Realms is pretty much devoid of the canonical NPC's (Elminster, Khelben, etc) - Not even mentioned in the book? is that the case?
Not mentioned at all that I saw.
5) Poster map-is one included?
Yes