1. Ignore oneworld, for UK-US they tend to focus on the joint business instead. You wont have any issue at all rebooking onto any JB route or carrier. This typically rules out AS, but given that (presumably) your original flight was also AS this may be slightly easier here than usual. They have to get you there somehow, but you may find you have to negotiate exact routing based on availability so best to have a few up your sleeve.
2. Yes
3. No