1. Sensitive
2. "At the top" is an idiom meaning the people who are the leaders of the group. So if someone very high in a business changes his attitude about something, then everything can change.
3. The word 'opened' in the sentence, "didn't want the envelope opened," feels more like an adjective to me or at the very least, an intransitive verb. Both sound fine to me as a native speaker. Of course, I am one of those people that strongly believes that, when it comes to English, as long as the meaning is clear and not filled with fads, it is acceptable.
4. I believe it should be, "Little did I expect to be treated like this." I'm not sure if that was a typo from you or your source.