I think you have been more than reasonable with this person and he is just giving you the run around wasting your time and money.
It’s common property which they have no right to so get in somebody to remove the shed (the OC will have to pay the cost initially) but then invoice the owner for the cost of the removal (I believe you must leave the dismantled shed on his property but advise him that if it is an eyesore and he doesn’t remove it within x days then you will engage a tradesman to remove it and again bill him the cost).
If they refuse to pay the invoice they are then non financial and so ineligible to vote at a GM. Even if the owner then tries to take you to NCAT you have a ruling against them and 2 fire safety orders which I don’t believe even the most incompetent adjudicator would overturn.