I suggest you install a test discussion list to experiment.
In the General Configuration, there is an option for where to direct replies, and I think an option to override the From: address. It's pretty flexible, so it's easy for all replies to go to the poster.
When managing users, you can turn on moderation, so their posts to the mailing list go into a queue. For your setup, this is more like a black hole, since none of their replies will ever get posted to the list. Give your main user permission to post without the Moderation flag turned on.