So I just booked the following itinerary through AC:

DEN-YYZ-FRA-PRG and back. DEN-YYZ and YYZ-FRA on AC, FRA-PRG on LH; on the return it's PRG-FRA on LH, FRA-YYZ on AC and YYZ-DEN on UA, all in Y.

After reading the MSC rule, I am quite confused. Apparently, LH is now allowing a small sports bag to be checked for free alongside your standard 23-kg 1 piece on the return trip and I am trying to find out whether I will be allowed that extra bag without a fee. The question is, under which airline's policies does my return journey fall, since the itinerary terminates in the U.S. and therefore new rules apply (US DOT). I see nothing definitive on the booking confirmation.

Any ideas? CA's extra bag fee has just increased to $70 on 6/1. If I have to pay the extra bag fee I might as well prepare a full 23kg bag (never a shortage of stuff to bring back, where are the good old days with 2x32kg checked bag allowance?).