Is it permissible to say: "So-and-so taught me" or "So-and-so treated me," knowing that they are merely means? And is that considered disbelief (kufr)? And is it correct to use the word "physician" (tabib) for human beings?

There is no harm in stating that a created being teaches or treats people, as long as it is believed that he is merely a means. Allah has described His Messenger, peace and blessings be upon him, as teaching, and the Prophet, peace and blessings be upon him, encouraged rewarding those who do good. Furthermore, the description of a created being as a physician has appeared in several texts, such as the story of the Prophet sending a physician to Ubayy ibn Ka'b, and his saying, peace and blessings be upon him: "Any physician who practices medicine among a people... he is liable."