There isn't any duty to press charges when accusing someone of having committed a crime, at least in the U.S., so (b) is unrelated. As for (a), it's obviously t

There isn't any duty to press charges when accusing someone of having committed a crime, at least in the U.S., so (b) is unrelated. As for (a), it's obviously the case that if they were convicted of a crime, it couldn't possibly be defamatory to accuse them of it; but it doesn't follow that if they haven't been convicted, the accusation is defamatory, because a lack of conviction is not the same as a finding of innocence (even an acquittal is not a finding of innocence, since it can come simply due to lack of sufficient evidence). A person alleging that accusations of their criminal behavior are defamatory cannot allege merely that they have not been convicted, but must show that they also did not commit the crime. There is no legal presumption of innocence when attempting to suppress third-party speech on the question of your innocence: if you want to sue someone for defamation, you now carry the burden of proof to show that what they claimed is false. (The only place a presumption of innocence comes in is when the government attempts to jail or fine you; then, the burden of proof falls on them.) It's quite possible, due to lacking or conflicting evidence, for the question of someone's guilt to be basically "uncertain", in which case they probably can't be convicted, but they also probably can't win a defamation suit. See, e.g. O.J. Simpson, where being acquitted of murder didn't stop anyone from continuing to accuse him publicly of it.