A lot of times, codeshare flights will restrict stopover rules. Codeshared flights often must be booked in married segments. Hence, the next connecting flight must be taken within 4 hrs domestic or 24 hrs internationally.

Reading further, I see this circle-trip will not price correctly. The distance between ONT and GRU is longer than either CWL-ONT or GRU-CWL. Hence, you cannot invoke the stopover rule for circle-trip or an open-jaw pricing. Also, you may bump into the higher-intermediary-point rule for GRU-CWL because of connection in ATL, JFK or AMS.

The best workaround this would be an end-on-end ticketing for 2 open-jaws. Ticket A: CWL-AMS-connect somewhere USA-ONT, open jaw, JFK-CDG-CWL. Ticket B: ONT-ATL-GRU-JFK. You should always avoid backtracking your open jaw connecting cities. Ticket A should not be CWL-AMS-CWL, AMS-ONT plus ATL-AMS. Neither should ticket B route ONT-ATL-GRU-ATL. The system will try to calculate roundtrips from common connecting point and add one-way segments before or after the roundtrip. After both itineraries are ticketed, call up the airline and have them string them together by cross-referencing each other.

Best of luck. Contact me privately should you require more assistance. I have extensive experience in ticketing circle-trip, open-jaw and end-on-end itineraries.