Greetings,
Other things tried:
-I've read about a workaround where you unplug and plug in the displayport connector, but there isn't any real details on it, and what i've tried hasn't worked.
How I got this to work was to setup the Eyefinity group regardless of what the monitors looked like (ie the DP monitor in an incorrect resolution because the CCC wouldn't let me choose the monitors native resolution no matter what I tried), then unplug and plug the adapter back in with everything running, which then suddenly let the monitor use its full resolution, at which point I could finish setting up the group. I did have to unplug and plug it in twice and it worked on the second time. Not sure that will help you alot here though.
I find the CCC / driver setup very challenging and often proves to be random attempts of trying things rather than a set sequence. Hopefully someone else may have had this exact problem before to help you more.