Flash commander mode?

So does 600 have a proper wireless one built-in or just a more formal optical that HAS to use the onboard one to trigger others?

It has a proper Commander mode. Just so you know... Nikon's CLS system is not wireless, from a radio perspective. It uses a system of flashes and Infrared signaling to communicate with Nikon's speedlites.

IR sounds better than simple optical slave, so I'll hope that my YN565EX has that IR pickup built in. Not a huge concern, but figured I'd ask.

Some of the Nikon Speedlites have a straight optical mode... and your Nikon compatible Yongue should respond to the camera's flash. If you search the forum's past posts, you'll see that flash has been reviewed/mention frequently as an inexpensive alternative...