Interesting one. You would like to think that being option C the Contractor is coming up with the most effective way of managing the project which will benefit both Parties and maximise both Parties gain share (or limit pain-share). I would get round the table and discuss if you think that these other people can cover those duties effectively.
Not withstanding that, you could instruct them to have a single person - particularly when that was a specific key person. They are obliged to issue a replacement and you have to accept if they are equally qualified or experienced. You could review disallowed cost definition and see if any costs being incurred for not replacing the Key Person would be applicable.
Above all - both contractual and non-contractual just talk and try to come to a mutual agreement on how this role would be best filled.
Not withstanding that, you could instruct them to have a single person - particularly when that was a specific key person. They are obliged to issue a replacement and you have to accept if they are equally qualified or experienced. You could review disallowed cost definition and see if any costs being incurred for not replacing the Key Person would be applicable.
Above all - both contractual and non-contractual just talk and try to come to a mutual agreement on how this role would be best filled.