Sorry, I can ony answer you the first question (biology isn't my science, any other but not that one, lol)
Dr. rer. nat = Doktor rerum naturalium = He's got a doctor in nature science (so like biology... as he should, lol)
Dr. Med. Hab. = Doktor Medizin habilitation = Habitilation is the highest university graduation you can get, with it you got the right to teach in university (if I remember it's meaning correctly)