Chapter Text
Between Dumbledore and his manipulations and sycophants as well as the last 2 Dark Lords and the devestation they caused in both the Muggle and Wizarding Worlds, Magic and Death realize that the Wizarding community in the UK is too close to being exposed which could very well lead to the extermination of all magical beings worldwide. As this would throw off the natural balance, the whole world would be in danger of collapse. They decide to send back Severus Snape.
In order to prepare him for his tasks, they spend a lot of time counseling him through his trauma, his own actions, the betrayals he experienced as well as the ones he caused, his insecurities, his guilt, etc. They discuss the people in his life and whether or not those people truly cared for him or were just using him. ***Note: While I think Lily was a friend when they were kids, I believe the relationship was unbalanced with Lily having most of the "power" due to Severus' desperation to have a friend and his lack of security and positivity at home. ***NOTE: The following is headcanon for the purpose of this story.*** During their Hogwarts years, she becomes more and more self-centered and withdraws from Severus except when it comes to schooling and how he can help her. When we see the scene where the Marauders have Severus upside down (with his underwear showing according to the book), she might have her wand out, but she isn't doing anything to help Severus at all. She seems to find it amusing and is practically flirting with James.
As part of the preparation for Severus, once the counseling is complete, Magic and Death take all of Severus' memories after the 5th year incident and suppress the emotions down to a manageable dullness so that he retains the information but is able to grab this new chance with both hands without his guilt and regrets crippling him.
They send him back just after he sees Lily smile, and he passes out before he calls her a mudblood. He wakes up days later in the infirmary after giving his mind and body a chance to settle. It's the day they board the Express to go home for the summer.
Going forward:
1. Before leaving Hogwarts, he sneaks up to the Room of Requirement and gets the diadem.
2. At the train station, Mary McDonald is standing with Lily speaking to the Evans family. Severus walks up and apologizes to Mary for laughing about what Mulciber did to her. Until the Marauders had him upside down, helpless, and threatening to pull his underwear off in front of everyone, he didn't realize how traumatizing the whole situation was. To prove his sincerity, he offers to be her Champion and duel the next person who attempts to hurt her.
3. When Lily tries to act like they are still friends, he will remind her that he saw her smile at his humiliation and that she has made it quite clear over the last year or two that she no longer considers him a friend but only a useful acquaintance. (Points 2 and 3 happen in front of the Evans family, including Petunia, and many others at the train station are shamelessly listening in. The news will spread quickly, and the Evans parents are going to have a loooong talk with Lily.)
4. Severus does not ride home with the Evans family as usual. Instead, he takes the Knight Bus to the Leaky Cauldron and goes to Gringotts. As his mother has passed away a few months before, Magic and Death told him that he could claim the Prince estate as the last of that line.
A. He takes up the Prince Lordship
B. He requests a meeting with Chief Ragnock and his advisors (this could take place that day or a few days from now)
C. Upon meeting, he presents them with evidence that he is there on behalf of Magic and Death and requests an unbreakable vow to keep all information discussed in this meeting secret.
D. He tells them about the time travel and the horcruxes. He gives them the diadem and contracts with them to have them hunt down and destroy all others.
E. He requests their advice and help in undermining both Dumbledore and Voldemort. I'm thinking he can use muggle printing (photocopying in the '70s) to write up the truth about Tom Marvelo Riddle and The Life and Lies of Albus Dumbledore and secretly paste them up with muggle glue all over Diagon Alley, Knockturn Alley, Hogsmeade, the Ministry, etc. Using the Muggle method means no one can trace it back to Severus, but knowing so many details will make people think that the author is a pureblood or well-connected, older halfblood.
While he could sit his NEWTS early and get a jump start on his potions mastery, he goes back to Hogwarts in nice clothes, his body and hair made healthy by potions and better nutrition, with his Lordship made known, and no longer giving Lily the time of day nor bothering to take notice of the Marauders (he's got jewelry on with protective runes to keep the Marauders from sneaking up on him and hurting him. Additionally, his years of teaching in the old timeline ensured he mastered an unimpressed facial expression and attitude which he puts to good use). He also demonstrates a sort of stern kindness and protectiveness over the other students, especially the younger years, no matter the house they're in. This starts a slow but steady trickle effect that will lead to better inter-house relations.
Dumbledore is fighting off the negative press that came about due to the information posted and is suffering with whatever consequences you the writer thinks works best. When he notices the change in Severus, he tries to win him over to "the Greater Good" only to be publicly slapped down. He also finds that Severus' mind is well-protected.
Voldemort doesn't realize his horcruxes are being destroyed, but he is dealing with more and more weakness. He begins to believe that it is due to the number of horcruxes he's made and decides not to make any more. He's also dealing with the loss of faith his Death Eaters have in him after the information was posted everywhere.
Severus and Lily never regain their friendship, but they do treat each other respectfully.
Lily still marries James and has Harry. The prophecy is still made. When Voldemort attacks the Potters, he doesn't realize all of his horcruxes have been destroyed. When Lily's protection on Harry bounces the Killing Curse back at him, he is fully destroyed fulfilling the prophecy.
Dumbledore is still an idiot manipulator who thinks Voldemort will return and wants to shape Harry in a deliberately abusive environment, but Severus (who didn't save the Potters because the attack took place at a different time than in the first go-around...also, he didn't give Voldemort the prophecy, so he wasn't expecting the attack to happen) uses his political clout to ensure Harry's safety and makes sure Harry winds up in a good home (maybe with the Longbottom's). Dumbledore (who deliberately ensured Voldemort was told about the prophecy by Pettigrew) keeps trying to get control over Harry, but decisive action is taken by Severus and others to take Dumbledore out if play permanently by forcing him into a Vow of non-interference that will cost him his Magic and/or life if he sticks his nose into anyone's business but his own. With Dumbledore hobbled, the whole Boy Who Lived nonsense is squashed, Lily's sacrificial spell/runes/ward that protected Harry becomes common knowledge (Lily acted as Harry's 'Hand' which fulfilled the prophecy), and Harry grows in the loving home of the Longbottom's. He is not spoiled or arrogant but a kind, level-headed kid. Severus also ensures Sirius gets a fair trial, thereby earning a life debt from Sirius as it was made obvious that Dumbledore and the Ministry were just going to toss him into Azkaban with no trial. Severus uses the life debt to force Sirius to go to a mental health professional on a regular basis (remember, Sirus also grew up in an abusive home). Remus decides to see one, too, in solidarity. As these 2 men do love Harry, this is the redemption arc for the remaining Marauders.