Judging by comments from the GI leak, "players must travel the world for a bit, learn about shouting," before they can start dawnguard and information from e3, namely that we fight a dragon who will teach us a new shout, makes it seem like players must be forced to start the main quest before doing dawnguard. This is something I really would not like to be forced into doing.
Then the DLC is not for you. Simple as that.
They are telling a story, and if you refuse to follow it, then that is your problem, not Bethesda's
Why cant I say no to when asked to go see the Jarl of Whiterun?
You can. Its called ignoring the quest. When someone wants to see you, you have the freedom of ignoring them.
I think I heard that you don't use the same elder scroll from the MQ. Don't take my word on it though.
Yes. They said you travel into the soul place to obtain a scroll.