I understand Lewis to be saying that you shouldn't revel in revelations of wrongdoing or cling to them in the face of evidence, because your duty as a Christian is to find a way to forgive even your enemies; that you should hate (his word) the wrongdoing itself, not giving it an inch, but be careful not to needlessly attach it to specific people without cause, or to allow such an attachment to impede your process of forgiving them.
Then, he's not saying that you should, on spec, be skeptical of claims of wrongdoing, or especially generous to mitigating claims. One way you know that's the opposite of his intent is that the essay is framed around the crimes of the Nazis; there is no possible way Lewis is suggesting we should be wary of news stories about the Nazis. Again: Nazism is a backdrop for not just the essay, or Christian Behavior, but really most of Mere Christianity.
This is a thread where really all we have to go on is a story about whether a media claim about wrongdoing is true, or can be mitigated. The quote then reads as an affirmation of the story! The quote would be saying exactly the thing I just said I believe it doesn't say.
But Lewis' quote doesn't apply here at all, unless you not only strip it of its context, but also turn it against that context. Nobody (at least, nobody identified) is reveling in what may have happened at the residential schools, or using it as a device to avoid forgiving any particular person. Our interest in the truth of what happened, and even our skepticism of mitigating claims, is totally independent of our duty to forgive, just as Lewis points out that our willingness to punish (or even kill) transgressors is independent of our duty to forgive them.
I think the whole thing is a lot clearer if you include the paragraph before and after the super-popular "newspaper article" quote.
I'm not clear on how I misread the original intent of the quote! I'm still interested in hearing that. So I'm re-explaining my own original intent in responding to it, but wouldn't want to be construed as doubling down on it.
My personal stake in this discussion is: I think Mere Christianity is pretty neat, and I'm attentive of the ways it gets used in online discussions. I don't know enough about Canadian history to have a stake in the truth of the residential school claims, though note that there is an expert rebuttal of these Quillette claims, from an archeological authority, downthread.