You 'could' galvanise those Cllrs that feel aggrieved by the action of the PC chair and chair of staff committee into a code of conduct complaint for their behaviour on the basis that they may be considered to have:
2.1 - failed to treat others with respect - by not actioning the correct procedures and excluding other members
2.5 - conducted themselves in a manner which is contrary to the Councils duty to promote and maintain high standards of conduct - by failing to address the matter in a timely fashion in accordance with established procedures
2.7 - has compromised or attempted to compromise the impartiality of anyone who works for or on behalf of the council - maybe...
2.9 - prevented someone from getting information to which they are entitled by law - response to complainant (a bit tenuous)
2.10 - bringing their office or council into disrepute - it is adversely impacting public perception of the PC
2.11 - improperly using their position to confer an advantage or disadvantage on themselves or another - self explanatory
2.14 - has failed to give reasons for making a decision - again, self explanatory
Not suggesting this would, of itself, resolve the external complaint issue but it might break the stalemate and result in action rather than inaction...
Rather depends how assiduous your particular MO is though....