Finally, it is highly doubtful that any Chabadnik believes the Rebbe is the Ribbono Shel Olam
i think you are right.
but they have substituted the Rebbe for HaKodeshBorchu and daven to the Rebbe directly, depend on him for their welfare and parnassah, and have transferred their emunah to him.
similar to the process by which the sun and other messengers of Hashem became avodah Zarah