I’ll offer my 2 cents’ worth.
You ask if Council approval was required?
The easiest way to find out is to ask your Council
(a) If approval for such works is needed. Or more correctly, “was needed” at the time the works were done; and
(b) If Council approval was needed, it’s an easy job for the Council to itemise DA and BA applications by a strata scheme and share with you whatever papers were lodged and whatever the Council decided. If approval was needed, it would be the strata scheme in its name who needs to seek approval and not an individual lot owner.
Once you establish the above facts, you are then better able to tackle your situation.