OP: In what programs do you have frequent-flyer miles? Just BA and VS? Other programs too? Do you have elite status in any FFP?

If you decide to use miles and can find suitable redemption flights now for the trans-Atlantic components of your trip, I would feel comfortable booking those now, and sorting out the internal U.S. flights -- or car rental -- later on. For the one-hour MIA-EYW flight, I would not worry about booking a premium cabin. But unless you have oneworld status, a stand-alone Coach ticket on an AA domestic flight will not include any free checked baggage.

If you are going to redeem miles, note that you can book one-ways with BA Avios (and maybe with VS miles, too), so you do not necessarily have to use the same carrier in both directions. Also, redeeming BA Avios for LHR-MAD-MIA on IB will cost a few more miles, but should have much lower carrier surcharges.

Note that the only airline currently offering EYW-MCO nonstop service is Silver Airways. AA and DL would each require a connection (in MIA or ATL, respectively).