Hello Charles. Nice to hear from you. I have several suggestions:
First, it looks like you are working from the first edition, titled "Making Documentary Films and Reality Videos." I brought out a second edition a few years ago. The word "Reality" has been removed from the title (because of 'reality television," and it says "Second Edition" on the cover. If you are able, I recommend you get the second edition because I go into the B-roll problem in more detail.
Second,put a white on black graphic somewhere close to the start of the documentary, explaining that you were unable to film activities of the sect.
Third, if there is a structure of some sort--building, compound, fence, sign, church, etc.--associated with the sect, film that to represent the institution of the sect.
Fourth, if you have not already shot your interviews, film each interview in a setting that somehow represents the personality and beliefs of the person being interviewed. Light and compose for emotional impact. Shoot visual footage of each person doing whatever they do, and especially things that relate to why they became a part of, and why they left, the sect.
Fifth, try to think of different visual metaphors you can film that can represent what each person is talking about to use as illustrative or explanatory footage.
Sixth, if you can't show a (bad) thing that was going on in the sect, can you show its opposite, a (good) thing, which is now accessible to the interviewee? e.g., cut off from friends and family vs. being with friends and family.
Seventh, edit tightly. Build the story from sound bites, but don't be afraid to let a narrator add detail that the interviewees leave out. The toughest thing with editing interviews is learning how much can be left out without harming the story. The tendency is to say, "I need this whole 90-second bite," when you probably don't.
Hope this helps. Good luck with the project.