OP can you clarify your routing on the return leg as it's not totally clear to me
If LHR-LAX is on AA then the strike won't affect that flight so the issue is CDG-LHR and that BA flight cancelled?
Would a CDG-USA*-LAX on AA be an alternative you could put to Expedia (who incidently have been sent rebooking guidance by BA which includes reboking on to AA).
* I see from wiki page for CDG that AA flies to JFK.MIA,DFW and PHL so could be plenty of scope to get you home that way
As flyerco says the expedia agent is takling absolute nonsense. A cancelled return does not invalidate an out bound.
Are you sure your LAX -CDG flight is operated ay Air Thati Nui - it's a little off their normal route network! More likely a code share on AA operated metal in which case you'd be checking in with AA