I found the unlock to cure Sarena to become human again.
However to get this text you have to avoid asking her "Have you ever thought about a cure?"
You have to completely follow through her entire quest lines before clicking the above question.
Sarena will tell you some stuff "i wont give away her conversation in spoilers" but she disappears for a few days.
After 3 days she returns purified and even better then before. I wont spoil anymore but she does have more random conversation lines if you take her along with you after shes been cured of vampirism.
I was only able to unlock this on xbox and get the mew text options to the cure question after completing 100% of all Seranas quests.
So the quests that appear about marriage and about if she ever feels lonley i accidently clicked those - So on playthru number 7 later today i will avoid clicking on those two questions and save them for the end like I did the "cure" question.
If Bethesda said she is not mara amulet available somewhere - then ive missed reading it.
but I will try again on number 7 playthru of Dawnguard.
Btw - Serana was cured on my human/warewolf dawnguard faction playthu.
Her different text replies after being cured are absolute Gold ~. Thankyou bethesda

i hope sarena can be married too in game. if she can be cured why not mara amulet too.