At this point, why do not the former saint's knights just tell everyone that the current witch was actually the former saint, infected by whatever curse the witch has? That way, they could prevent the current saint from becoming the next witch.
Perhaps many would not believe it, but if it happened repeatedly, they probably would.
Just because the former knights tell people something, doesn't mean it is true, or believed to be true. The saint is a hero beloved by everybody. Believing that someone so pure could become her evil nemesis would be a tough pill to swallow.
How the reveal would go:
The new witch is the former saint.
And that's not even touching how the current(fake) saint, who mightve been the best that ever was, is a fake. Who would believe she is just some random girl taken by mistake. It's like believing superman is just some ordinary human.
And let's say for one moment everybody believed what the former knights told them.
That doesn't change that the former saint wants to destroy the world.
And it would be entirely reasonable for people to want to kill the current saint (especially after beating the current witch) so she can't become a witch. Kill both of them, and maybe the cycle ends.(the "good" end from the game)
Elrise has reason to keep her mouth shut since she is specifically trying to use game knowledge of events, while also working her own plan to change things.
The knights have reason to keep their mouth shut because:
If nobody knows they are working with the witch/former saint, nobody can capture them and force them to betray her and get her killed. And as long as they help her stay alive, the current saint won't become the new witch.
It's a win win. Hide the witch and protect her. Meanwhile OP Elrise protects humanity and doesn't become the worst witch.(presumed, nobody knows elrise is fake). Humanity has a certain level of peace as the witch is limited in what she can do out in the open due to Elrise being so OP. And the knights simply keep their loyalty to the one they served for so long.
In terms of trying to prevent the current Saint from becoming the next witch.
Without real spoilers, let's just say that most of the things you are probably
thinking they should try, has already been tried or will be tried.
Also a bit of a spoiler, the knights and Elrise
are not the only ones who know the truth about saints becoming witches. There just is
nothing they can do to stop it.
This series for the style it has been written as(kinda casual), has the most consistent and well done worldbuilding and plot I have ever read. It's kinda like buying a low budget direct to DVD movie by some nobody actors/directors, and finding it holds its own among even blockbusters.
Everything makes sense in the end. Everything has a solid explanation.
Whether that will properly translate to the manga adaptation, I have no clue.
I don't usually like to engage in conversations on stories I have "read ahead" on(adaptations usually), because I don't want to ruin the theorizing and speculation.(unless someone specifically wants spoilers)
But I do it on this series, because it isn't like the other series where people make the dumb choice for no reason, or certain things are forced, or things are never explained and make no sense.
My point with everything here, is simply try to think of reasons why characters would act or do certain things. Think of ways the problem might be solved, and ways that solution could fail or backfire. You just might find yourself surprised and satisfied as some of those things happen.