I think I faced exactly the same thing as you yesterday. I got an error message where it said something about the audio, sorry don't remember the error message. I tried a few things and gave up and just installed everything (Raspbian and Retropie) again. It should be another way though..... Its absurd you have to reinstall an OS because of this....
I connected it to the second port and booted it up, then started playing around with the config again. Lo and behold, hdmi_group=1 didn't cause a catastrophic error! The rest was cake, thanks to you guys.
Very much appreciated, thank you so much for all your help!
Good to know. I tried a handful of games a while back and didn't have any luck myself, but it makes sense that some of the less demanding games would be playable. What kind of frame rates were you getting?