You can achieve what you want if I have understood by the following
Install the private groups plugin
a. Public discussion – just set up a public forum, and all users will be able to access this – no private groups settings needed
b. Then for the keymaster only forum – do as follows
1. Go into dashboard>settings>bbp private groups>topic permissions and activate this feature
2. Create a group in dashboard>settings>bbp private groups>Group 1 (or whichever group you want)
3. Create a new forum, set it as private (so only logged in users see it) and in the forum groups set group 1 with access
4. Then click save and when then page refreshes, you will see topic permissions have appeared below the forum groups
5. Now select Create/Edit OWN topics
This then means that within that forum, users will only see topics they have created, and be able to reply. The keymaster can then see/reply to these topics. Any topic created by the keymaster will be visible to all users in this forum
So from the user point of view they get a forum where they can post threads which only you and they see.
I think this should achieve what you want – and given they can create multiple threads on what looks like to them as their own private forum, maybe is even better as they can create a new thread for a new topic !!
Nice explanation. Thank you.