Pierre,
There isn't really a specific custom role you should use. I would create one for these users (one for each set of queues that need to be separately accessed.)
In terms of coaching..
The Listen right / button allows a user to hear the call.
The Coach right / button allows a user to speak to only the agent
The Join right / button allows a user to "conference in" to the call (both ends can hear them.)
HTH