1. The point is well taken. Dealing through opaque third-party vendors, even if well-known, carries significant risks and this is one. OP's ticket is SNA-ARN and that is the legal obligation. The fact that he had something scheduled at DTW only matters if he actually scheduled a stop-over at DTW. It is unlikely that AA would care about that nicety, but Expedia does. Unless the savings are enormous, it's not even worth questioning.

2. As others note, AA is the operating carrier, not BA. While it is highly unlikely that AA will touch the ticket, it is worth a call. OP will need to make his case in 2-3 brief sentences and hope that he gets a sympathetic agent. The worst he faces is a "no" and he is no worse off than he is now. Best to call during regular business hours in the US. Use Skype or equivalent to call US customer service.

3. If OP can get over the hump of AA making a change, AA may well make cost-free changes for him.

4. If this does not work now, do not make a fuss. Once OP has flown outbound to the US, he should call AA back and start again. While AA should still not touch a ticket absent IRROPS, it might.