A nation or society governed by civilized, compassionate people who place a high value on human life & ethical conduct does not choose to cover up something like this when they know the coverup itself will cause countless #'s of deaths, with the potential to plunge all of humanity into a modern-day plague, destroying economies, lives, etc.
But with the communist government of China we are looking at one that doesn't seem to think twice about murdering their own citizens for the sake of their own power. You think such people give a damn what happens to other country's citizens? Do you think them incapable of researching and trying to produce a biological weapon for which they also try to produce an anti-viral so they could severely weaken other nations while selectively issuing the anti-virus to use as leverage to gain ever more power?
I'm not suggesting this is what happened or that it could not be one of the other possible scenarios you mentioned. But when taken in context including their willingness to plunge the world into this chaos by denying, delaying & obfuscating facts, refusing help from other country's scientists regarded as premier experts, etc., murdering those of their own medical community who dare to tell the truth, the other possibilities seem to be far less likely.