Boy, it sounds like you are being way too nice. You have done everything by the book. There are orders for the shed to be removed. It is unequivocally established as necessary to comply with the OC’s safety obligations. Keeping the shed has been put to the OC at a general meeting and the motion was lost.
In your letter, I would have reiterated all that and said that the EC had an absolute obligation to remove the shed without delay and that if he did not do so within the 14 days, the EC would arrange for it to be removed and the unit owner would be charged for the costs involved.
With the benefit of hindsight, it would have been good to have asked for orders that the OC could remove the shed and charge costs to the owner if the owner did not remove the shed within some reasonable period.
I think the main thing now is to have clear protection for the OC if it removes the shed. I would be looking for the order to include that you can do that. However, if you did just remove the shed, I don’t think the lot owner would have much of a case against you given two lots of fire safety and council advice, a tribunal order that it be removed, and the failure of an OC resolution to grant exclusive use to the lot owner.