Update on this. We have no real timing issues anymore as AA canceled the later ATL-ORD flight and put us on a midday flight. We'll have about 4 hours in the Flagship Lounge.
However, my oldest is flying from BDL and meeting us at ORD to join us on our QR legs. AA has also canceled her nonstop from BDL and put her on a connection through DCA that doesn't get her to ORD until 4:15pm. I'm not willing to risk that so I can just cancel her AA legs and start her award ticket in ORD. If I do that and get her to ORD on UA (nonstop arrives around 1pm), I assume she'll just have to go to T5 and wait for QR check-in to open to check her bag? Then she can go to T3 and meet us at the Flagship Lounge. Is there any possibility of her checking her bag with AA as it's an AA award ticket? I assume not if there's no AA leg. If so, does QR check-in open exactly three hours before the flight? Thanks.
No, if she's flying UA to ORD and then QR from ORD on a separate ticket then AA doesn't have anything to do with bags. She'll have to claim her bag at ORD then take it to T5 to check in with QR. I'm not sure what UA's policy is with regards to checking bags through across separate tickets, but I suppose you could always check on that as well (look on the UA forum, etc.). QR check in at ORD should be open 3 hours before departure but that's something you would need to double check with QR on as well.