You pose interesting arguments and question.
You state "Your argument assumes that the ethical status of an action depends on whether or not it is specified in a code of ethics and on the frequency of its commission."
However, you need to be aware that I am looking at this though a very narrow spectrum, that of the legal system (and I am including disciplinary actions in that spectrum). If you are asking from a "moral" standpoint, then my answer would be quite different from the "legal" standpoint. On a personal level I disagree with many laws and the interpretation of them by the courts and other authorities. However, I have to deal with them as they are interpreted by those in power and not on my personal beliefs. In addition, I have to also view them through my own experiences in the system.
The primary reason that I believe the results would be as I point out is that the court (or other authorities) will view this with a risk vs. harm analysis. As I mentioned, most medical care providers have a very strict set of rules regarding the observation of patients following any type of procedure or medication. I'm not personally familiar with the drug Propofol and have never been involved in a case involving that specific drug. However, I have read about it and know that it has a number of side effects including a possible reduction in blood pressure, seizures, etc.
However, the medical providers normally do not break the medications down in their guidelines they simply state that a patient must be observed and accompanied after certain procedures and coloscopies, as well as the scope that goes down the throat, are two of these that are normally included.
In the risk vs. harm analysis, the only harm to the patient from being observed is that of social discomfort. There is no medical harm that will result. On the other hand, if the nurse did not observe you and you suddenly suffered a seizure or a drop in blood pressure, or any other of a myriad of possibilities, then you could easily suffer a dramatic injury by striking your head or some other body part on the sink, toilet, etc.
Since hospitals and nursing homes are sued on a regular basis for the failure to restrain, failure to use bed rails, and failure to monitor patients, it is obvious that these type of incidents do occur with some frequency.
I do believe that it is extremely hard, and to a degree unconstitutional, to hold a nurse responsible for an ethical violation that is not clearly spelled out. Since the licensing boards are either a governmental entity or a quasi-governmental entity then they must operate with an eye toward due process and punishing a nurse for an ethical violation that is not clearly spelled out would not be upheld by the courts.
Is it possible that the hospital does not have a policy regarding watching people who are recovering for surgery/procedures? Sure, it's possible but it's unlikely. In addition, it is also unlikely a hospital would ever hold a nurse accountable for erring on the side of caution.
I have represented nurses as they appeared before the boards before for complaints and, in my capacity as a plaintiff's lawyer and as a trial consultant, I have been involved with filing complaints against nurses. None of them were under your exact facts, but some of them did involve falls when they shouldn't have occurred and so I am familiar with a slight variation on your facts.
If you would like, I can get a couple of other experts to read over this and see if they agree with my assessments but I'm certain of my position and arguments. It doesn't mean you can't file a complaint anyway but I am quite certain that if I, or anyone that has done trial law, were hired on this type of case then the nurse would not be punished.
That is also supported by the statements you received form the Nurse Administrator at the clinic and the Division of Health Services. I am relatively sure that when you hear back from the State Board of Nursing they are going to tell you the same thing.
In addition, should you visit with a local attorney that handles personal injury or malpractice, the two areas of the law that most directly apply to this, that they are going to tell you the same thing, although they will also cover the "lack of damages" aspect of the cae which really has nothing to do with whether there was a violation or not.