Re: to disable posting to parent forum (only post to subforms)
Sub-forums each have their own permissions page which can override the global permissions.
For each subforum: go to the subforum. go to options > users > change permissions, change things from inherit to modified and save it, so it keeps the current permissions for the subforum.
Then you can go to the homepage/parent forum, go to the permissions page and disable posting.
Yes, if you change the application type to mixed, everything should still be there.