I don't know why your Owners Corporation (O/C) drafted a new By-Law, but if it hadn't been Registered and attached to your Plan's Title Documents at the time that your dissenting neighbour lodged her Application with the Office of Fair Trading, they (OFT) would have no way of knowing that your works had received the O/C's prior authorisation.
OFT could of course have attempted contact with your Strata Manager or the Secretary of your Executive Committee, but as you've correctly observed they're not the most efficient of the Government's Bureaucracies!
Sorry, but unless you want to spend your money on litigation instead of on something positive like your new bathroom, I'd just move-on and put the whole episode down to experience.