Iberia is responsible for rebooking you on the cancelled flight since it is so far out. It can take some time for the ticket to confirm.
For your bike, I don't know anything about declaring it in advance. You will have to pay twice, once each way, and it's with the airline who is operating your first flight.
so Iberia for the 2 flights from Bxl to Dallas (as first flight is Iberia) and AA for the 2 flights from Dallas to (as first flight is AAback home)? Why is Iberia trying to push me to AA and giving different messages to me during these last days?