The sentence is
I believe that であり may be being used here to connect the two verbs together but I'm not certain.
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であり is the continuative form of である (a literary and stiff copula), so it's used to connect verbs in a broad sense. However, you have missed ながら, which in this context means "albeit/despite/although". 当事者でありながら means "although she is a person involved (in the matter)".
Michiko, who has been left out of the conversation despite being an involved person