What a mess.
The short answer to your question is no, the OC can’t force you to remove the staircase, they don’t have the power to do that themselves. For example they could pass a resolution that an owner must remove the stairs, but they can’t enter the lot and remove them, and if the owner did not remove them, then they have to take further steps to enforce the resolution.
There are a number of things that may compel an owner to remove the staircase in these circumstances:
1. an order of an adjudicator or the Tribunal
2. a Court order (although I am not sure which Court would have jurisdiction)
3. a Council order.
In the third case, the OC would have to lodge a complaint with Council that a lot had illegal stairs, that the stairs were in breach of Council requirements and requesting that the Council take enforcement action. Then the Council would have to decide to take that action and issue an order for removal of the stairs.
In certain circumstances the owner may be able to appeal a decision to make an order.
I am baffled by the Council refusing copies being made on the basis of copyright law, I have never had that issue with our Council. There was a High Court case in 2007 which confirmed that an owner of a property could use plans forming part of a development consent, even though that owner was not the architect’s client. Copying plans on file at the Council would constitute reasonable use.