BTW, I've finished the main quest of Dawnguard as a Vampire Lord (a Breton mage named Neclord.)
The three questions are:
1) Is there safe storage in Castle Volkihar? If so, where?
2) Is it possible to re-enter the Soul Cairn? I still have a few optional quests there left open.
3) Finally, I am considering using my Vampire Lord build as a Dark Brotherhood agent. His method of assassination: pick lock in the middle of the night, enter house, bite contract victim (recharging Neclord and completing the contract), and leave (hopefully without being seen). Is that an effective way to operate as a vampire assassin?
Re: safe storage, you should check out the Notes section of the UESP on the Keep.
Also if you exited the castle and Valerica's Study via the balcony when you left the first time, you can fast travel back to it on your map and reenter the castle there. Once you open the door the SC is right there.