In these circumstances, whomever the buyer is wouldn’t have to worry about OC approval once they had bought the unit, because they didn’t put the staircase in. The OC can only take action against the person who did it. That still leaves the lack of Council approval.
I would still be wary, just because I can’t stand things being a bit uncertain. I am familiar with one case where an owner did not get Council approval for a certain type of use of their lot. This has caused both them and the OC a lot of grief over a considerable period of time, and in the end the owner has had a large amount of value sliced off their property.