I think we've hit the crux of the question, Dave: Whether the method of calculation for a refund, subsequent to a carrier cancelling a confirmed ticketed segment for its own business reasons, is the "sum of the full fares" as provided in fare rules (for voluntary cancellations), or whether some other method of calculation of the refund is required in this circumstance. I still don't feel I know the answer -- and I am not convinced that the fare rules for voluntary cancellations truly dictate procedures in case of involuntary cancellations. But in any event it sounds like it's time to write to CX, see what they say, and then proceed from there.