Whatever you do, do not go ahead with any internal changes without approval! If you do and your OC takes you to the CTTT I believe you could be ordered to return unit to original condition, reversing any works done. You could be held accountable for all manner of things.
Best for you to take the matter further in the correct manner. You have the engineers report. Do you need council approval? Get all the info you need to state your case. From this forum, Fair Trading, etc. Research what you can and cannot do.
Apartments/units have to be renovated sometime! People can't live with pink toilets or orange ktchen benchtops forever! If an owner covers all the bases, crosses the t's and dots the i's, I don't see how an OC can reasonably deny renos.