You have to remember you are dealing with a government. They will let limit access to what information you want, what areas you you go to do your research, what patients or doctors you talk to etc. You are at the mercy of these people.
Also note, the doctors from WHO are of all manner of nationalities. As a government you cannot discount spies. Especially a paranoid gava like china.
You can only ask for the information. You can ask for permission to perform research and you will have to state what areas you want to go, what exactly you want to do etc.
As far as research goes, you have to base your information on information that can be cited or evidenced credibly. How does WHO give recommendations to nations of the world based on some information that shows up twitter or a chinese citizen claiming X and the govt says something else? You can have your suspicions but if you do not have concrete evidence then it cant be in any reports made in an official capacity.