BD is set 20 years after DD. The stories are linked, but it is not a direct continuation. The baby at the end of DD is involved with the plot of BD, being exiled to that realm by the Divine One. There is a novella included with the UK version of the game (and the fan pack: novella and audio CD) which describes some of the intervening events, and a brief synopsis was posted in the forum some time ago.
In any case, you do not need to have played DD to follow BD's story, though there are references to DD and some familiar characters and places if you have.
If you move your characters by clicking and holding the mouse button, rather than repeated clicking, the screne will automatically recentre. Also, there were party selection and screen recentering hotkeys added after the demo was released. I found these hotkeys made a big improvement in gameplay, and after getting use to being able to moving the screen focus off the main character by bumping the edge of the screen, I did so by accident much less frequently.
