hi Brian, i had a feeling you had your finger on top of the problem. you can consider putting rubbing alcohol on the model and then dusting on the powder to help with adhesion.
here are two basic how-to videos that can help with different pigment techniques:
http://www.youtube.com/watch?v=iGA32YSK9Uw&feature=related http://www.youtube.com/watch?v=nx1e6ooh8PA as for the dead figure, first i really like that you didn't do a big pool of blood. we can have a long discussion about the reality of war, the use of gore and how it is to be handled in a diorama. my opinion is to "symbolize" that a soldier has been wounded or killed but to stray away from depicting bright red crime scene splatter. just my view, what would be consider a romanticization of war and one i can understand is idealistic.
second, Jeremy pointed out all the great things about the pose, especially the tilted head. this ties in with the road weathering. if there were more debris and gravel the downed figure could be nestled in this rubble. perhaps the one thing that Jeremy has mentioned is the symmetrical arms and legs. had one leg been bent or an arm crooked it would create more visual interest.
one thing you can still do even though the scene is finished is take the dead soldier and run his back on some sandpaper to flatten him down so he sits even better on the base.
third point, you asked what can be done to add weathering to the road. consider you used a light buff tan as a wash for between the cobblestone. close to the sidewalk and in some areas here and there add a wash made from Raw Umber and Burnt Sienna mixed 70/30 to make it appear like some areas have dirt and some have dust. you can check out my attempt at cobblestone
here or search flickr.com for pics of cobblestone which gave me inspiration.
again, great scene and look forward to your next project. cheers, mh.