How can a prorated refund be calculated based on subtracting the cash value of the already-flown segments? That's counterintuitive I'd say. I believe there's a general IATA rule that explains how to handle voluntary (deducting the value of flown segments as if a ticket was bought as such) vs. involuntary refund (by fare component first, and prorated to mileage if only part of the fare component is flown). I saw it somewhere before but am unable to find the exact rule again. If anyone has it, that would be great.

xONEx has only one fare component for the entire trip, so if flights are cancelled and you are eligible for an involuntary refund (which AA still provides if no alternative within 4 hours I believe), the refund would be the fare will be prorated to mileage + unused taxes.