Re: Auto-complete stopped working
haloid – I wrote a plugin for group invitations that uses the same autocomplete code as the messaging system to allow group mods to invite any member to a group: https://wordpress.org/extend/plugins/invite-anyone/. So it’s definitely possible to extend BP to do what you’re asking. I haven’t looked at the code for messaging, but if it’s anything like the invite code, it’d be pretty easy to do. A quick glance at bp-messages.php makes it look like you could probably alter BP to do what you want it to do by finding the function messages_ajax_autocomplete_results in bp-messages.php, commenting out the code that populates $friends, and replacing it with a list of all site members (you could use exactly the code I use in the Invite Anyone plugin).
Better still would be to build it into a plugin. Wouldn’t be too hard. If I get a chance in the next few weeks, I might take a look at it.