NWA never had any options on the 900 pre 9/11. They were CRJ200 options that they likely switched to the 900. Either way, it's a moot point, because there is still no way for one single airline to bring on 72 airplanes in about a year and a half. Nobody really knows why they split the order, but I suspect ramp up time had a huge bearing.

As for why Compass operates on the A concourse. NWA does the ground handling for Compass, so that's a huge reason for the A concourse. Not to mention the 175 doesn't fit on the C (the 900 as well for that matter) concourse, the 900s are all on B and Mesaba runs the ground service on B and C.