I anticipate a few possibly short scenes where Harry does basically the same thing and manipulates Bones and Madam Longbottom into supporting him as well. If this scene is being held as evidence of sexism, the laws of evidence tell me those scenes should be held as evidence against, but somehow I don't see it happening that way.

Actually, the law is that the average scene that isn't Harry manipulating Lucius must be evidence against sexism, not that any such scene is evidence against sexism. Many of the possible scenes in the set {Harry doesn't manipulate Lucius} are sexist.