I'm very familiar with the DG questline from the anti-vampire side of things but I have never done the Civil War questline. Your question is rather difficult to decipher but I think you are asking about the quest contained in this link: http://www.uesp.net/wiki/Skyrim:Prophet_%28Dawnguard%29
If so, and you're having trouble getting info from folks in Dragonbridge, I bet you can simply just go directly to the location detailed in the link and still complete the quest.
Hope that helps.
I haven't run into that problem but you don't need to question the townspeople for that quest. Just go south along the, y'know, dragon bridge and you'll find the next objective along the road.
Are you wearing a Stormcloak uniform? In my experience, as long as you are NOT wearing anyone's standard uniform, you can go pretty much anywhere without being hassled about your allegiance in the civil war.