Why on Earth would someone like to be considered a native speaker? I deeply respect foreigners who speak some Russian. They can enjoy life in Moscow, they can read Pushkin in the original, they can laugh at Russian jokes. They can get a well-paid position in a company that works with Russian colleages, after all. Isn't that worth the effort?

Most people have only one native language anyway. It wouldn't hurt to be a non-native speaker of some other languages.

unfortunately always be there someone who says you aren't a native spearker (actually this is true) even if you speak better than they. Anyway I think if you have a certificate it should be valid to states that you have the qualities for teaching.