If the SWU had cleared on AA 973 then they can book you in the cabin your are booked in. If you are in Y on AA 973 they cannot book you into J on non AA metal no matter how much space is available.
Incorrect. AA tries to protect members upgraded on AA on alternate flights. It's much more likely they will do so on JB partners such as BA or JL. See post 20 by
JonNYC. It's certainly what I've experienced, too.