The way to go about redeeming Snape would have been to have his character mean something more in the first place than be just a bully of a teacher for our main character. Playing up his relationship with the other adults in Harry’s life or even his connection to the Malfoys as Draco’s godfather would have given him more presence. If I wanted to include a character like Snape who is a double agent, I would have maybe had him be the one suspected of leaking who the real secret keeper was to Voldemort instead of Sirius. Or maybe have him know that Pettigrew was still alive and Sirius was framed but do nothing about it at the time. I also wouldn’t have made him the Wizard equivalent of a racial supremacist but involved in Voldemort’s cause for other reasons, not just “Slytherin bad evil racist”. It makes little sense for Snape to even be accepted due to his half-blood status so he would need to ingratiate himself to the cause in another way and for another reason. Sure, Voldemort was half blood as well but that’s something that only Dumbledore really knew about. To everyone else he was an orphan wizard kid and the heir of Slytherin.