I could actually imagine the opposite scenario. As the OP mentions a partial dispute, I imagine that this means disputing the amount roughly corresponding to the return part of the original ticket. That is what you do under the notion that a good or service has not been delivered, and in that case, what had to be delivered was a plane ticket. In a way, perversely, if you accept that you have not been provided with the valid title of transportation that you had requested by AA, you might sadly be exonerating BA from EC261 as the latter only apply with a valid ticket. They might, however, not pick up on that loophole or even be aware of the charge dispute.

Like you, I'm glad the OP got back to us with an update, but also a bit sorry that they did so to effectively only update their own train of thought rather than pick up on - virtually - any of the questions asked by those who wanted to help him/her.

To add to PUCCI GALORE's excellent post, I would say that the main organ of communication is not the mouth but the ear, and that it is actually easier to fix communication issues if your meaning is sometimes unclear but you go out of your way to hear what others say, than if you express yourself perfectly clearly but remain impermeable to the meaning of others.