I’m no lawyer, but I think what your OC has done is perfectly permissable.
The Act says that there must be a motion on the agenda for the AGM to set the number of EC members and a motion to elect them. These two motions are ordinary resolutions not special resolutions. And they don’t necessarily have to be in that order.
Whilst the Act, schedule 3 clause 4(2), also says that the OC (not the EC) must fill a vacancy on the EC, there is nothing to say the OC can’t hold an EGM, which you apparently did, to re-set the number of EC members, reducing it by 1 so that a new election was not required.
Any GM (AGM or EGM) can rescind or amend an ordinary resolution passed at a previous GM by way of another ordinary resolution.
Two more points:
1) For a block of 12, an EC size of 3 is probably sufficient, unless there are hordes of octagenarians dying (?) to get on the EC, in which case 5 should be more than sufficient.
2) There are no Strata Police driving around raiding random blocks, checking that everything has been done absolutely correctly, and it doesn’t sound like you have any owner who will complain to the CTTT that he/she was denied their rightful place on the EC.