2)Let's look at that age difference, shall we? Japan is very strict about the ages at which children can begin a certain school year, which usually begins around April 5th. Even if a child is born on April 6th and is a bonafide genius, they're out of luck and have to wait a year.
From background details, eg Homura's calender, the foliage, etc… it is extremely likely that the series takes place towards the very beginning of the school year.
Mami is in 9th grade, for which you must be 14. Unless her birthday is extremely soon after term starts, she's still 14 during the series. If her birthday squeaked in just before the start of term, she's just barely 14 and is probably technically only a few months older than the rest of the Mitakahara 5. At the oldest, she cannot possibly be 16, which is not an adult, and hence cannot by any recognizable definition of the word, be a pedophile.
But we're looking at what the age difference actually is, so, continuing on…
Nagisa isn't shown in a middle school uniform, so we can assume she hasn't started yet. Since you have to be 12 to begin 7th grade (first year of middle school in Japan), we know she isn't 13 yet.
However, Kyubey targets girls going through PUBERTY, so it's highly unlikely she's much younger, either.