The difficulty is that the outcome you want, ( a higher MTOW) may not be possible to achieve. If the aircraft hasn't been properly tested, documented, permitted in the country it was built then CASA and RAAus can't accept it. The fact that they made a mistake by letting it fly at a higher weight doesn't mean that once the have realized this they can continue to let it fly at the higher weight. The only outcome you are likely to get might be some money that represents the difference in value between a high and low MTOW. I.e. The difference between what you paid and what you could sell it for now because that really is the only real loss you can demonstrate.
Getting the lawyers involved might speed things up if what you are asking for is possible and simple. If you are asking the impossible then their only option is to fight back with lawyers.