1) a chuunin can become an anbu then later an anbu leader if you are good enough. (ex itachi)
2) calling someone sempai its just a respect thing, it has little to do with ranks. In some cases, a jounin can call a chuunin sempai.
3) raidou wasnt an anbu, he got owned by oro's kunai while wearing the chuunin/ jounin vest.
4) yes kakashi was an anbu. theres a picture of him in anbu clothing with his dogs.