This should explain the contamination question the best I believe.
"By far, the best time to dry hop is after most or all fermentation had taken place. Once wort has fermented into beer, it has become a more biologically stable. For example, after fermentation is complete, beer is anaerobic, meaning there is no oxygen dissolved in the beer. Many beer spoilage organisms are aerobic, and in the absence of oxygen, these spoilage organisms remain permanently dormant. Second, the beer now contains alcohol, which further inhibits spoilage organisms. Finally, the process of yeast fermentation drops the beer's pH, making the environment inhospitably to the bad guys. Three strikes and you're out."
source
"http://www.boomchugalug.com/dry_hopping.htm