Let me add another perspective to the discussion on prefixes for the months. After reading your post, I remembered attending a presentation on "Time" in the planetarium of my home university. The director had input relative to this discussion so I emailed him for clarification of his position. He responded as follows:
"Sept. has always been the 7th month. July & August originally had Latin names for 5th and 6th months till they were renamed to satisfy the egos of Julius & Augustus. They were also originally 30-day months, but J & A ripped off days from February and appended them to their months, also to satisfy their sense of self-importance. They stole from the end of February because it was the last month of the year. March was the first month because the year began with the vernal equinox. Our switch to a January start came in 1752 when the British Empire finally adopted the Gregorian Calendar even though it was promulgated by the Pope, who of course was an evil Catholic."
I suppose there are a number of stories surrounding the debate of where the prefixes came from (historically) -- perhaps many of us will never be sure -- but there will always be someone willing to share another version.