As facilitator, I would pick b) , but I do not think they could even react if they wanted to, they would be plain witnesses.
c) would be possible but not polite
by picking a) (which I do not think is possible without a dedicated policy) I wonder how their contribution could go beyond simple reactions...
art 3.3.1 would still give you some tools:
two of them may still be re-invited by the Rep and Lead, or a policy could be proposed to generalize the problem, while still keeping in mind the issue related to the kind of tensions they could process or help process.