There seems to me to be 3 ways to play through Dawnguard. And each of those ways seems to have some different paths forking...

1. As a Dawnguard, soul-trapped to get through soul cairn.
2. As a Dawnguard, turned vampire for soul cairn then back to normal.
3. As a Dawnguard, but gone over to vampires.

In scenario 1, is it possible to get Serana to be cured? (I think that's dialogue based if you stay Dawnguard) Can you get Valerica to come back? Can you get Serana to 'turn' you?

In scenario 2, how do you get back into soul cairn? (having Serana 'turn' you doesn't do it.) If you get Serana to 'turn' you, can she then be talked into getting cured?

In scenario 3, is it possible to get Serana to be cured?

Other scenarios that I've missed?

To get back in the Soul Cairn you needed to go out on the balcony leading into the Portal room to unlock the fast travel marker on aforementioned balcony.
You can do this before or after entering the Cairn portal but from what I understand is you can't later and you will never get acces again without mods or console commands.
Then you can travel on into the Cairn any time you like, without needing to be a vampire at all and no more weakened soul on your next visit.
My follower didn't follow me in though as she wasn't a vampire.

I didn't find the weakened soul to be any kind of handicap, was lever 40 odds when I first went in though.