It doesn't make any sense to me. I have no idea what "individual dumping margin" might mean.

It means, if companies don't cooperate anymore, they still can get an invidual dumping margin, depend on the authenticity of data and on the exception of a single country ( here mean how a country react to the dumping of that company ). I think, hope can help!!! :">

This is my best guess. How much a extra waste a company can dump into the environment will be determined. They will use data to determine this, probably provided by that company. However, it may be that the company provides false data. If that is the case, the company may not be allowed any extra dumping, or very little. Some countries do not follow these rules, so the company may provide no data from a single country if that country is not cooperating with these efforts.

Damned if I know.

Exactly. While pls kindly tell me the structure of the sentence with an exception of a single country . In my opinion, the whole sentence means if one of hte cooperative companies refuses to cooperate, different countries will have the same margin and the margin will be up to the authenticity of the data. I dont know where I am wrong.