Originally Posted by
Leopold
That is correct.
The U.S. regulations do not require airlines to follow the "most significant carrier" rule. The IATA does, however. Under that rule, AA is the MSC because of the LHR-DFW flight. Stopovers make zero difference, I.e., are wholly irrelevant.
No it isn't . It would be the MSC if the passenger was connecting. Since the 1st part of journey is simply within Europe on BA, then BA's rules apply
There is no MSC calculation to be made since there is only 1 carrier and 1 sector
If the journey was to anywhere other than US, then the baggage allowance would be recalculated at each check in; since it is a trip which includes the USA, then the baggage allowance in force at the start of the journey applies to the whole journey