yes, you can create a group, where only members of the group can post, however it is a custom code modification to assign people automatically to the group after joining the website
other website members can follow the group and won't be able to post anything in there, just viewing
Group turns into read-only also considered as a custom code modification.
Yes, you can choose users who will be a participant, but first you need to befriend with them and choose from the friends list.
My advice would be to download oxwall and install it, its free what do you have to lose. And it will actually save you time because if it works for you, then you wont have to waste time with something else.
So give it a try, just install it in a test subdomain, have your tech people do this for you and then play with it and see if it will works. If you need a little tweek here or there, its possible.
I think it will work for most of what you want, and the rest will come in time.