I like what you’ve done in this topic, @mattdm . Congrats 
After reading the whole topic, this would be the potential suggestions which came to my mind:
There could be 2 higher ranking clickable titles for (1) the “public” section and (2) the “private” section. Both of these would have a “create” button, which could stay always apparent (or not). It could remain a + sign or be more prominent, like a “create” or “new” button.
The “personal chat” collapsible section could get a hiding pencil button like “categories” and “tags”, which would direct to the homepage of chat. Then this section works the same way as the categories and tags ones. It makes sense to me: You have the people you chat regularly with, the button lets you edit that list, and 1 additional click for a new chat is available from there.
The “New”, “unread” and “sent” links appearing in the sidebar when you click “inbox” aren’t really necessary and could be discarded (or not). If it feels bizarre to only have the “Message” clickable title and directly the collapsible “personal chat” section under it, a permanent “Sent” could be put there. This would be the equivalent of the “My posts” in the public section. A “More” could also be added, if there is some use for it.