I don't think that you will be able to book directly with AA> Looks like you are trying to price an itinerary that includes travel A-C via B as the sum of A-B plus B-C fares rather than an A-C fare

If Expedia is able to book it as 2 separate fares and it is saving such a significant amount then book through Expedia

Assuming this is a published fare and not a special fare that Expedia is offering, the fare rules regarding refunds and changes will be the same as if booked through AA.