This is a very tricky situation you are in. On a previous trip LGW-DUB-AUH-BKK-HKT,(strange routing i know, but cheap ticket bought through Etihad) the first leg being Air Lingus. Even though i was in business and have a gold card with Etihad, i was charged for extra weight. I protested this, saying Etihad own a % of Air Lingus and this was a code share flight and that my weight allowance, should be allowed...They would not budge on this and i payed the excess. I then got hit again on the BKK-HKT route even though the bag went straight through.( much too heavy for Bangkok Airways domestic, was the reply)..Etihad have now increased their economy and business class baggage allowance to make matters worse ( to compete with Emirates), so check with each airline, and try and get it in writing if poss. Good luck