It is pretty far fetched. Think about the value proposition vs the risk of exposure of such a scheme. Blackmail is a major felony, and deliberately choosing not to report illegal content of this nature is as well. Why would Facebook do this when they are already swimming in ad revenue?
Yes it is.
No, no, big public businesses like facebook routinely use evidence of child sexual abuse to coerce adults into doing their nefarious bidding. The complete lack of evidence is in itself proof of how effective their methods have been!