Didn’t Robin say she thought it was her brother-in-law? If mom didn’t care about being outed why didn’t she confide with the woman (now bestie) she helped lock up?
I don’t know and neither do you. There are zero good reasons for the FBI to name the informant in the case and plenty of good reasons not to.
Good chance the informant’s name is in the trial transcript. I would think defense lawyers would ask for the name. And she could have testified. I would hope the filmmakers read the transcript or at least some of it to confirm the name. Why does the FBI care about a case 18 years ago? The only way they would care if it was being appealed and it’s not. And like I said a retired FBI why would he care?
I doubt it, the confidential informant was likely the catalyst for the investigation, but after bringing it to the attention of the FBI, doesn’t seem to have played a role. The FBI would still care because if it was in the habit of giving up the names of informants on TV shows, it would be a huge disincentive to become a confidential informant.
How do you know she played no role after giving the info? I don’t know and neither do you. 
Did you read the trial transcript?
I did just watch a six hour documentary on the subject and listened to the accompanying podcast and not only did they never mention that the informant played a role in the trial, but the FBI is still protecting his/her identity, it seems likely that I’m correct, but this seems important to you so you’re right everyone at the FBI is just being silly to protect the identity.
I am still wondering why you think her own son lied about outing her? And “she likes the other DIL better” is not a very good reason to me.