Unfortunately there is no specific mention that I could find that says that marco polo benefits apply across the whole journey when CX is the most siginificant carrier.
That's an interesting question. Is it that the published baggage rules of the Most Significant Carrier that apply, or the published baggage rules plus any benefits extended by the Most Significant Carrier to the particular passenger?
I suppose the other question is whether this is being taken up by other airlines? IATA resolution or not, I've not seen any word from anyone else that they will be moving to the MSC approach. As far as BA is concerned they might continue to apply their existing rule of first operating carrier.