You are correct. The past perfect tense is used when talking about things that happened before something else that happened in the past. The present perfect is used to talk about the past without referencing a specific time.

You are right. Both sentences relate to a viewpoint in the present moment, so they should both be in the present perfect. The second one is also a little odd. 'Imagine' doesn't make a lot of sense in this context : it would be more normal to say, for example, 'I can't believe'. Where did you see these sentences?

It would be good to see more context with the first sentence as there are instances when it could make sense.. e.g. when you are telling a story.. This guy thought he knew everything about Japanese. But I knew what he was saying was wrong. For the past 10 years of my career, I had been teaching foreigners how to learn Japanese. So I let him finish what he was saying and then I pointed out that.. Just to add, the original sentence itself is a little odd. I would simply say.. For the past 10 years, I had been teaching foreigners Japanese.