let me answer your questions:
It is possible to support single user by multiple supporters, but not simultaneously. For example the first supporter can leave the conversation with user after solving her issue. When the user then writes another message to the conversation, the conversation is available for all supporters as a new pending support request, so another supporter can take it. Unfortunately, the scenario with two supporters communicating with the user at the same time in one conversation is not supported and I think it won't be an easy customization.
All chat requests are queued, but every supporter can choose which one they will take. So there is no number that can be displayed to the user. But you can do some customization and count all pending and active support requests and insert a custom message to the conversation informing about number of currently waiting clients.
There are only two states of support chat - at least one supporter is available (that means they are logged in support chat) or no supporter is available and user then cannot use support chat for contacting support. As I mentioned above, you can do customization and insert a message informing the user about support unavailability when there are currently at least X pending requests.
Please feel free to contact me if you need further assistance.