This is may be a judgment call, but my strong inclination would be to make a single filing including all the information. It appears that that broker is the hub and the applicants are just the spokes of the wheel. The intent would be to help FinCEN see the big picture; the pattern of activity.
If the SSNs are fraudulent, it is highly unlikely the individuals exist. Nevertheless, if you give FinCEN all the information it will still end up in their database, which is the ultimate objective.
When you tally your advice here and develop a plan, then I suggest you call FinCEN and tell them what you are going to do rather than ask them. (Most of us can agree to a plan easier than we can make one.) Make a memorandum of the conversation, enclosing a copy with the SAR and keeping the original as part of your supporting documentation.