You can recove your soul after accomplishing your objective in the Soul Cairn, Valerica will give you the quest, it is fairly trivial.
Should you become a vampire, you will need to be cured in order to continue working with the Dawnguard, you'll get the quest to get cured from members of the Dawnguard.
Interestingly enough, getting your soul back will net you 1 black soul gem, while getting cured of vampirism will cost you 1 full black soul gem.
Answer to this is Probably Not
Aela has a strong loyalty to Jorrvaskr. Which has its loyalty to the Imperials. If the Imperials win then Vignar Gray-Mane is the new Jarl of Whiterun.
She is part of many factions but these are of importance :
- GuardFaction (makes an actor a guard)
- PotentialMarriageFaction
- TownWhiterunFaction
She is part of Town Whiterun faction and Guard faction. Which automatically makes her not available for marriage in your case, since entire of Whiterun will not have Whiterun guards. They are replaced by Stormcloak soldiers.
Also according to her entry at UESP:
Aela is part of the Guard Faction. There is a bug wherein due to being part of the Guard faction, Aela may behave erratically if she's following you when beginning the Battle For Whiterun quest.
According to entry for Battle For Whiterun at UESP Stormcloak version:
Also during Battle For Whiterun (Stormcloak version), Aela the Huntress may disappear if she is your wife at the time. She may come back eventually after you moved on to the next Quest, Liberation of Skyrim.
Since you have completed Liberation of Skyrim quest and still can not find Aela, you probably might not be able to marry her.
Best Answer
As of right now, it is not possible to marry Serana. See the link to the Elder Scrolls Wiki as well as discussion forum posts here and here on this very topic.
http://elderscrolls.wikia.com/wiki/Serana#Trivia
As to will it ever be possible, anything is possible. You are certainly not the only person who has suggested this, and it is not unreasonable that this option could be patched into the game in the future. There is even a petition here to encourage Bethesda to add this option. As of right now though, there are no plans to make marriage an option.