One bit of advice I can tell you is be well prepared. Be prepared for the characters to go off on a complete tangent as to where you want them to go at a particular time or place. Characters playing can do something that can totally goof up all the well laid plans and make you go back to the chalk board to try to sort out where you need them to go and then how to get them there. That is what makes RPG so much fun. Good luck with your scenario
You may have problems getting everyone to start elaborate in depth characters.
It takes a while to fill out all of that background. Sounds like a solid premise though.
As stated I would try and be ready for players, with background of your world / realm.
I say familiarize yourself with your niche / strengths. Create an environment were those work for you and the pc's.
I agree. A lot of the hurdle of player participation and especially initial incentive and initiative itself, is uncertainty of setting - as a GM, for Oli and KN's games, I wasn't really sure exactly what setting they were using, so I just took an educated guess and went with what I felt were the most likely, based on the religions that were allowed, or which would be "generic" enough that I could create a background and area for my half-orc rogue from another area, that I was later able to retcon and place into an actual location within the setting to make it "official".
With some foundation, I think players are much more likely to feel comfortable with coming up with ideas and writing up backgrounds and histories and locations for their characters, so you may want to provide at least a good guideline of the setting, rough area and time and maybe major religions, features, maybe even events and nations of note, etc so they can have something to build on or from - I think that could help quite a bit.
I understand that question was aimed more at KNtoran, I'm sure he will respond. I want to toss in my own opinion though.
For me this sort of thing happens all of the time, and I mean all of the time. No matter how well you plan out something or how detailed you set up an environment the pc's will always come up with something you have not foreseen.
In these instances, which will happen, the only defense is to be comfortable with your plot. GM's always have the story playing out in different paths in their heads. This is good and you should be formulating all of that when applicable. However don't beat yourself up if it takes a turn you did not see coming. This is when your preparations shine. When players go off the beaten path is when all those fundamental foundations of the plot come to life.
You cease to have a dry well written plot and begin to have a plot that is unfolding via the players. This transformation lets the players know they are there in the thick of it. It tells them they matter and gives them a sense of ownership which by the way is a prerequisite for character development and an above par game.