The outbound is not invalid simply cause of flight cancelled on return. Whoever at Expedia said that doesn't have a clue. HUACA. Sadly Expedia agents are regularly this misinformed. (Its not them being incompetent, just lacking training/having been badly trained)
If you had booked directly with AA then AA could rebook you. As it stands,you can fly out and show up for return. BA then must rebook you. Only issue is if space is available. Thus best to get done before hand. As this is a simple TATL booking, Expedia should move you to another BA or AA flight. It doesn't matter which, they're in a joint venture. Thus revenue/expense wise there's no difference which you fly on TATL. The issue will be getting Expedia to handle the rebooking. If they refuse call AA and ask if they can take over the ticket. There will likely be a fee (on DL it's $50), but then AA is the one handling everything, not Expedia.