I contacted the Outlaws and got a quick response from Scott.
The calibration issue is because the 990 apparently felt the crossover on my surrounds was set too high. Scott tells me: "Unfortunately, the Model 990's auto setup has a strange way of advising you that the x-over setting you have selected for the surround channels was less than optimal given the speaker's actual, measured frequency response and additional options for that channel built into the Model 990's bass management system. "
This would be fine if it were just some kind of warning, but it comes in as a hard error that doesn't let me save calibration settings. So I have to adjust the xovers to 80hz, auto-calibrate, then set the xovers back. Not a big deal, but it would be good for new owners to know up front.
As for my other question, no the 990 does not support a single rear surround for 6.1 like the 950 used to. Again, from Scott: "...the Model 990 can not specifically be configured for 6.1, though you can certainly connect the speaker directly to a single surround back channel (though you'll need to manually configure all of your channel levels and not use the auto setup function)."
-Joe