a) yes, I'm an agent
b) The first change of TC has to be on BA because only BA published the fare. If the first change of TC were on AA for instance, normally that would be fine as AA also has the same fares filed as BA for RTW (and others), but in this case AA did not have a fare published, only BA, so to get it to price out with the BA fare, BA has to be the marketing carrier for the most significant flight (first change of TC).