The answer to your question is that FBI informants can reasonably expect that they will be under surveillance. That is going to be the big kaboom in all of this. Carter Page, Paul Manafort, and Papa D. Were all part of FBI intelligence operations that were classified. The crime syndicate has used this method for some time, they will create operations, classify the truth, and then provide "evidence" that is heavily redacted to support a prosecution or media narrative.
Question - when the evidence that is exculpatory is classified above what a circuit court can reveal, or redactions are almost impossible to fight… How do you prove that you were doing what you were supposed to do for a classified operation you can't technically disclose? When an informant is charged for conspiracy or fraud based on their work for the intelligence agencies, who intervenes to say they are not guilty? Who makes sure there is no bias in this process?