On this playthrough I've chosen (as usual) to cure Serana of her vampirism, It's taken me about three days to return to Fort Dawnguard, and yep, she's there. Trouble is she's still a vampire and she's still wanting to be alone so she can go through with it? Sounds like a bug? If so, how do I fix it?

On a side not, Dexion only takes two of the scrolls, not the dragon one. Another bug? If so, how do I get rid of the dragon scroll?

Regarding Serana, no idea

Regarding scrolls, if you're looking to get rid permanently...the Dragon scroll can be sold to a certain magical librarian who is fond of atronachs